Is there a way to calculate the perfect angle of incidence of a horizontal stab for a given airspeed?
One clue may be increase in power and speed.  It is not unusual to trim down as plane accelerates from climb to cruise.  Since you are faster, the plane is generating more lift at a given AOA.  Greater speed will require more downtrim.  Your engine may be too powerful for your current airframe.

If the plane is safely flyable, you may try to isolate the problem by gliding power off to see if Hstab, elevator, and trim are working properly.  CG of course must be within limits.

If everything is ok, you may need a larger trim tab rather than resetting the Hstab angle.
Remember, the Hstab incidence compared to the wing AOA is critical.  I would not mess
with it.

If the problem isolates to the new engine, you may also check the thrust line.  A few degrees can make a big difference.  Your more powerful engine may need to be mounted with more downthrust.  That, and a larger trim tab, may do it.

I would definitely seek out people who have done similar modifications for expert advice.
Please, be safe.