How high do I mount my 70 Yamaha onto my 1986 15' Boston Whaler from the transom to the bracket on the motor?

And what is the proper pitch size to get maximum speed and efficiency? Motor mounted first time with cavitation plate to bottom of hull-no good. Motor then raised 1 1/2 inches-now it feels like motor is not getting to proper RPM's without trimming motor out of the water. Motor mounted twice by professionals-Now I wan't to do it myself!How high do I mount my 70 Yamaha onto my 1986 15' Boston Whaler from the transom to the bracket on the motor?
Best bet is to speak to boat manufactureres. Remember some years ago my dad had the same problem but manufacturers very helpful. Advised correct height above transom, distance offset (left or right) and prop size.