The best way is to use a flowmeter, Smartcraft, etc. to determine what your fuel usage is at given RPMs. Then you can calculate fuel usage vs. speed.
I think my best is at about 3500 RPMs which gives me about 55 MPH. According to my Diacom software, I'm burning about 14 GPH at 3500 rpm. So, that equals about 4 MPG. Not bad for a boat!