Take your speed, eg 30 mph, and divide it by your gph burn number, eg 10 gph, 30/10 = 3mpg. After a while, you just look at the gps for speed, and at the gph, and do the math in your head to come up with the most econmical milage.
Almost all 2 stroke motors get their best milage in the 3800-4200 range, so you select a prop, set the trim and tabs, move the coolers and occupants to get the best speed at that rpm.
Usually in doing this, you give up 1-3 mph on the top end, especially in the prop selection, but a boat is nothing but a floating compromise
. I went with a 4 blade powertec that gives me 27 mph at 3900 rpm burning 12.6 gph for about 2mpg, but lost 2mph on the wot speed.