i guestimated about 2 litres per hour at 2000 rpm - that's about 1 gal(us) per two hours - pretty close.
by adding rpm to your sheet, you won't be "guestimating" - you'll know what your consumption is at various power points.
what size engine do you have and what size boat? Do you tow a dinghy?
25hp. 33' boat is about 7-8ton all up. Tow dink sometimes.
My point was / is that by tracking the actual consumption you will get to know what *your boat* consumes.