You're totally overthinking the fuel flow meter. I have a stupid amount of money tied up in avionics and engine monitoring, but frankly now I just use the red knob and listen to the engine, and use the airspeed indicator. When you are LOP for a specific RPM, for a certain fuel burn, you will get a specific IAS, as all the fuel is getting converted into horsepower. Rich of peak, this is not the case, as a lot of fuel is going out the tailpipe unburnt, and the rate will be highly dependent on the mixture setting. If you run these engines below about 22" of MP, there's not a lot you can do to blow them up, so get up there and go fly LOP. Pretty soon you will know for what IAS how much fuel you will be burning, so your airspeed indicator will essentially become your fuel flow meter. If you keep your testing low, there will be bugger all time spent on climb and descent, so what you used on an hourly basis in cruise will pretty much reflect the burn rate you calculate at the pump at the end of the day. This will give you something as useful and more accurate than most fuel flow meters, not to mention that the off-the-shelf calibration of most fuel flow meters is terrible, so you have to calibrate them anyway, or not trust them. Regardless, your son has to build hours, so it may as well be spent doing something useful.
Tom. Again.