For me, I think it unlikely that a rough rule would work for all.
There are just so many variables.
Deep V hull or fast planing.
Length of Hull (More hull in the water = more drag)
2 or 4 Stroke.
RPM, (While 4 strokes use less fuel at lower RPM 2 Strokes can use more)
And swell size
I think the only sure fire way is to test your RIG. Get a feel of what it uses under whatever load (Also helps with early diags). Ask people who have the same hull and similar sized engines.
Just my opinion