I have built a stern drive hot rod from a outboard boat. Running a 300 he Chevy alpha. The trim is a just ed exactly to 21 inches at the trim pistons. If I trim up a touch more with the trail or switch I gain more speed. ? Is does the transom angle matter in the trim measurement. My transom is. 15 degrees angled back.
Most stern drive boats have a 14 degree transom and is considered "industry standard" so you are in good shape. From my 35 years as a stern drive shop owner, ideal trim moment is when there is no left or right feed back in the helm. Try trimming in and out and you will get the idea. Of course this will change if someone changes position in the boat or a throttle change.