I remember seeing a cool write up long ago to figure the best, but here's a text version
Put shock in lower mount and droop truck completely.
Extend shock fully and swing it in an arc fore and aft while marking a piece of cardboard with a sharpie through the upper loop.
Bottom rear end out metal to metal, bottom shock out and swing fore and aft with sharpie through upper loop on cardboard.
Where those two arcs cross on the cardboard is the optimized location for the given shock, lower mount location and vehicle.
Move inward 3-5 degrees and call it a day. Sometimes the vehicle doesnt allow this due to frame rails or other restrictions but from scratch, no obstructions thats the way to do it.
Aim for 90 degrees at full bump. That is from the front leaf spring pivot, to the lower shock pivot, and then to the upper shock pivot. This creates a slight rising rate.