I would have thought that if you work on 4 litres/hour (roughly 0.75 gall/hr) at cruising revs you won't be far out, although these figures are based on a Yanmar 3GM30F (30hp) in a previous yacht.
Count roughly 1 litre per hour per 10 hp, so flat out, you'll do 3 litres per hour, proportionally less at smaller throttle settings - handbook MIGHT give you power/rev curves to work this out from.
I have the same engine in a similar size boat. I use 2l/hr on average. If I push the engine to 2500 it uses 2.5 but normal cruising at 2200 (6.2 kts) it uses 1.8. This agrees with the figures from the previous post.