RE: Not wanting bellcrank
And to find that point one could just set the height at or level with the center line of the crankshaft or just a little below.
If you want to find it precisely cut, fabricate and install the leadout guides now in such a way that the airplane can be suspended by the guide above the left wing. Simply find the point on the plate that the airplane will hang perfectly straight down and that is where you make the guide holes.
Why you are at it I would make this plate adjustable fore and aft for flight trimming. The idea is to allow the lines to be adjusted from ninety degrees to fuselage axis to slightly aft of that point. In other words this allow the airplane to point slightly to the outside of the circle. This fore and aft adjustability of the guides will be important in trimming the airplane to stay on the lines, I.e. keeping it from chasing you across the circle.
John