How Accurate Is The Fuel Flow CAN bus data?

All flow meters are going to require calibration, Even the CAN bus if you are at all picky.

The CAN bus is going to report a K-factor of 3500 pulses per gallon.

But this assumes that all your fuel injectors were manufactured at Zero tolerance.

Your injectors might be a micron larger or smaller but all those microns add up when the engine injects 10,000 times a minute.

If you would be OK with being within a 1/2 gallon out of a 20-gallon tank (2.5%) then the listed calibration will be way more than accurate.

If you are looking for something more like 6 ounces (1/4%) out of a 20-gallon tank then a small effort will be needed.

It is not that difficult. Assuming a 20-gallon tank...

Fill the tank to a repeatable level and tell the Fuel gauge you have way more fuel than you really do. (30g)

Then fly the plane and keep refueling it WITHOUT resetting the Fuel gauge.

You do not need to fill to any particular level but you do need to know how much you added each time.  Don't spill any!!!

Record the amount of fuel used at each fill-up to the 1/100 of a gallon.   i.e   8.47g  9.23g, 

After you have refueled 3-4 times and used more than a full tank (20g) of gas, Fill it one last time to the same point that you did the first time.  9.60g  (Start with a full Tank and end with the same full tank.)

8.47g + 9.23g + 9.60g  = 27.3g

Assuming you fuel gage reports that you have 2.3 gallons remaining (27.7g used) and you actually have refilled 27.3g;

You will need to correct the K-Factor by 27.7/27.3 = 1.01465 ( ~1.47%) ... 

1.01465 * 3500 = 3551 New K-Factor.

You should now be accurate to better than 1/4%,  6 ounces per 20g tank full.

The engine should go silent within 2 minutes of when you expect it to, but I don't recommend testing that theory.  

Agree with Bill 100%

I have a Garmin G3X which uses the CAN bus data from the engine ECU and I believe cannot be calibrated.

I have been monitoring this my longest flight started with 179 L (47.3 US Gallon) and ended with 7 L the last hour close to home burning off fuel !. The Garmin and the physical measurement were different by 2L or 1.1 % .

pretty happy with that I would always fly with bigger Margins unless it was a test.