Gas usage on twin Merc 250 XL?

[SkaterRace]

I'm looking at a boat (wellcraft fisherman 262) that has twin Mercury Verado 250 XL and am wondering what I should expect for gas usage while cruising at around 30-40 mph and at trolling speed. I can't seem to find anything on their site about it and best I can find for the engines are the max fuel usage. Anyone with a similar setup have some averages or rough ideas I can base my decision on?

 

[Scott Danforth]

Assume 0.4# of fuel per HP per hour