Sig Kadet LT40 needed indeed about 6 degrees downthrust, with less it was gliding perfectly and as you add power it went looping, not enough (down)elevator to keep it level!

But here is what classic books says:

"The resultant of total lift and total drag must pass through point of intersection of propeller thrust and gravity force. If propeller thrust passes through CG, then the resultant of total lift and total drag must also pass CG."