You can build a spreadsheet in Excel and graph engine horsepower and ideal prop horsepower vs. rpm. Once you have the prop curve, you can also calculate the ideal fuel consumption vs rpm.

I have done this for my last boat and it was very close to actual fuel consumption. Remember, calculating fuel consumption between fill-ups is only an average value and includes both high and low speed operation where fuel consumption varies widely. The only way to get an accurate reading other than a Flowscan, is to operate for a long period of time at a sustained rpm, i.e. 10 hrs @ 1800 rpm, or draw from a measured container, i.e. 1 gal @ 1800 rpm for 'X' minutes.

bhp @ rpm [prop bhp] = (bhp/rpm^2.8) [where prop curve meets hp curve] * (rpm^2.8)

i.e., for 160 bhp @ 2600 rpm engine [assume prop curve meets here] bhp @ rpm [engine bhp] = 110 bph @ 1800 rpm [from manuf. curves] bhp @ rpm [prop bhp] = 160/(2600^2.8) * (1800^2.8) bhp @ rpm [prop bhp] = 57 bhp

NOTE: the "rpm^2.8" means "rpm raised to the 2.8 power".