I use a Navman 2100 it fits in the fuel line and measures the flow rate. It also calculates how much fuel is left in the tank. It is meant to be accurate to <5%. I have found it invaluable. Cost approx £125 from memory. Used on a 5.6 avon with a 4stroke Honda 90hp
It works like this:
1) Fill up the tank.
2) Set gauge to total fuel in tank (either through fuel pump reading if empty tank, or total size of tank) - Suggest downrating this to 90% of tank size for obvious reasons!
3) If you then add more fuel - then just adjust the current fuel level on the gauge.
It did prove my manual guage was accurate, and also shows current fuel rate. This is interesting to watch as you reduce the revs and watch the speed and fuel flow change (sometimes dramatically!)
2100 Manual available to download from:here
They also do a 3100 which has many more features (two fuel tanks, visual bar chart display) - uses a large square gauge as opposed to a small round one with the 2100. you may need to buy an additional flow sensor for two tanks though.
Any other questions just ask
p.s. I have no connection with navman other than being a satisfied customer!