At Antigua Race Week 2024, a boat protested another boat for not rounding the mark on the correct side. The protestor was not close enough to observe the distance between the lead boat and the mark, but observed that the lead boat looked like it had to alter course, was close and upon review the tracker clearly showed that the lead boat had not rounded the mark on the correct side. 
Witnesses on the mark boat  were unanimously prepared to testify that the lead boat had rounded the mark on the correct side and it was not close to contact. The protest was withdrawn without a hearing. 

There are doubtless many similar stories as @angelo described in his example above.

Different GPS devices have different degrees of accuracy. In the surveying and civil engineering industry there are gps + accelerometer devices costing many thousands of dollars with high degrees of accuracy of under a meter. In defense and aerospace there are even more expensive devices with even higher degrees of accuracy. 

We describe GPS sensors in two ways, precision and accuracy.  Accuracy is the degree of closeness of the GPS estimated location to its true, but unknown location. Precision is the distribution of the estimated locations around the mean estimated location (ie the density of observations).  The two most common measures to define accuracy are:
1.  Circular Error Probability (CEP) .  CEP refers to a circle in which 50% of the values occur. If a CEP of 1 meter is quoted then 50% of observed horizontal point positions should be within 1 meter of the true position.
2. R95 or 2DRMS ( 2*Distance Root Mean Squared) This is a 95% probability circle where 95% of the observed horizontal point positions will fall within the specified diameter around the true position. 

There are two manufacturers of Electronic Race Management Systems (ERMS) which provide virtual start lines, race management software and tracking. Vakaros with Race Sense and Velocitek with Race Puck.  There are numerous manufacturers of nautical GPS devices that can provide post race tracking, ranging in price from $400 to $5,000. Any assessment of tracking data as evidence in a protest will have to take account of the accuracy of the device providing the data.

Neither Vakaros nor Velocitek build their own GPS sensors. They use readily available 3rd party sensors with published CEP and R95.  

The OP referred to Race Sense.  Vakaros provides two gps devices with a considerable difference in accuracy. The most commonly used device is the Atlas 2. This is a stand alone GPS device without a Real Time Kinetic (RTK) positioning base station. Based on analysis in other forums (I own an Atlas 2 but I have not pulled it apart to look at the sensors), the COP is around 1 meter and the R95 is 2.13 meters.  The second device made by Vakaros is called a Helio. This utilizes an RTK base station which improves the R95 to well under a meter. Vakaros has not published the data on any of their devices but their marketing materials refer to centimeters on the Helio. That seems optimistic but it is doable with RTK technology. 

Velocitek's Race Puck is solely based on RTK technology. At the time of writing they haven't published their COP or R95 results but they say they will. 

The Atlas 2 accuracy is very credible for using for start lines for most racers. A COP of 1 meter and R95 of 2 meters is plenty good enough for amateur racing.  The start line is relatively stable , reinforced in some cases by linking the base station with a positioning services like Swift Navigation's Skylark.  At something like Sail GP they use much more expensive systems with multiple sensors on the boat and RTK base stations on a fixed onshore position.  

However, if you allow race sense data, or any tracking data, in the protest room, you need to be very careful that you understand the compounding error effect of comparing the estimated positions of two gps plots. If the COP is 1 meter, then there is only a 25% probability that both boats are within 1 meter of the distance between the two boats defined by the gps plot. The outer edges of the two R95 circles can be anywhere from 2 meters to 4 meters apart.

Here are some guidelines that occur to me (just as a starting point)

1. The PC should not look at GPS tracking before hearing testimony from the two parties, and other witnesses. This prevents the approximate tracking from GPS creating any pre-testimony biases or coloring the judgement of the jury before they have heard from the parties.
2. Before allowing any submission of tracking data, the judges must know the accuracy of the GPS device and understand any latency in the subsequent tracking data. Preferably, someone on the panel should be suitably qualified  so that they can convert the temptingly looking line on the data into visualizing the wider band that it actually is. 
3. Before looking at the data,  I recommend defining the question you want to answer. For example, if you are looking to define whether a boat was on port or starboard, or whether it was previously coming from astern, or tacked to leeward. These are questions gps data may be able to help on. If you are looking for a point of time when an overlap was established , or how close to leeward a boat tacked to leebow, then you should probably not even open the data unless it is RTK enabled and even then you want to know the accuracy of the device.

We havent talked about how the frame of the boat is defined by gps or the direction it is pointing, which a simple stand alone gps will not provide because it measures course over ground and not the direction the bow is pointing.   

 I hope that helps frame the discussion. 

This one is worth answering directly, because you and the app agree and it wasn't letting you see that.

The rule 14 card only ever proposes one move — you hold your course — and asks whether holding it is legal. So on a card where the answer is "that's a foul", the app is saying exactly what you were saying: swift and decisive action was required and holding on wasn't good enough. It just never gave you the words for it, and it never told you that contact was actually coming, which left "surely she'll get out of the way" as a perfectly reasonable read of the picture.

The card now says so in the setup: she is not keeping clear, and you are going to hit her. What's left to judge is the rules question — rule 14's first sentence binds both boats all the time, so the interesting half is the second: a right-of-way boat need not act until it is clear the other one isn't keeping clear. When that stops being true is a real question, and it's the one the card is trying to ask.

Clearly placed there by a trimmer trying to share the guilt! ;0))

Thanks very much John. I appreciate your efforts in replying in so much detail.

Our annual program includes a Pointscore Series (Pursuit format; scratch -> biased towards slower boats) run every fortnight ( 15 races per season), Championship (Yardsticks; 2 races each day; 14 per season) and Sprints (Yardstick, 3-5 races per day; 24 say races per season) => run in turn every other week.  (so C-P-S-P each month).
 
We'll be running our annual October Regatta (6 Divisions) , Hobie States, Spiral Nationals + the International Canoe World Titles this year so we'll be busy, and having lots of fun and enjoyable times out on the Bay!

As Race Secretary I want to apply a common entrant list and a common set of rules for our club races. (KISS).  We are transitioning from a Cox-Sprague system to low-point (Sailres) next season. We have a meeting next week to decide on the rules.

After much research and simulations and yours, and others kind feedback, my preferred approach is DNC= Starters+4. I thinks the committee could settle on DNC = [12] which is starters + 4 for our average fleet size of 8-10. There is a group who want to keep the regatta approach of Entrants + 1.
 
We'll see who 'wins' the debate. Interesting Times (pun intended)! 

Cheers and Thanks again for all the great feedback. 
PS many clubs face similar issues and so it is worth discussing it here.

Looks like several scoring systems allow for it.  You will find the instructions for how to do it on RacingRulesOfSailing.org here.