Assuming you have a great weather day with flat seas, I like to use the rule of thirds. 1/3 out, 1/3 back, 1/3 for reserve. You need to know your exact MPG to determine this. And there are many factors that influence your MPG. You need to determine this for yourself for your boat, with your engine, with your prop, with your load, in the sea state you expect to go offshore in (not inshore) etc. Don't go by internet #s or what others say they get, do the test yourself.
Mike
Hydrasport 2900VX
Seaduction MMSI 338-018-823