SIB's performance is maybe litle more sensitive to weight than hard boats, so how much fuel You need pends also on your load. Last summe did some longe distance boating with the bombard (420) and 25 hp. At moderate cruising speed and good conditions averaged 0,35 L/NM(2 not so heavy persons, some gear and fuel).
Adding one person, increased consumtion to 0,5 L/NM. Rough weather/ swell would increase consumtion further. These figures as i remember now but should be accaurate enough.
You have a bigger boat so I would fuel 20 Gal, then You should be on the safe side, and dont need to worry all the time.
fun on a boat is inversely proportional to size...sort of anyway