Re: Prop depth on Evinrude 7.5hp
That old style gearcase may limit the height.But raise as far as possible just before venting(cavitation) becomes a problem.Be sure it pumps water.
I'd be really interested to know how high it allows you to go.
I do know my 55 Gale 5 measures a full 17" from plate to where it rests on the transom.Similar leg to your 7.5.
Depending on your transom I would think that without modifying the transom you could raise it as far as the clamps allow.Just cut a piece of wood to steady the height while you tighten the clamps.You can get more formal once you reach the ideal height.I wouldn't think you would gain much speed but it could be the difference between planing with a load and not.