First off, there is nothing to keep you from fabricating more than one mounting point on the frame. Visualize the mounting brackets on a 4-link. The bracket could include holes for mounting the shock at 5, 10, 15 and 20 degrees for instance. You'd want the adjustable bracket on the frame instead of the control arm.

Determine how much wheel travel you will have in bump and droop and install your bump stops accordingly. Let's say you have 4" in bump and 4" in droop from the "at rest" position. Now, measure from the hat where the wheel bolts up to the pivot point on the lower control arm. Let's say that is 16". Now let's say you find a shock with a travel of 4". You now know that the shock will have to be installed at a distance of 8" from the pivot point of the lower control arm in order to use the shock's full 4" of travel at vertical (no lean). The more you lean the shock, the less travel it will have, so you have a built-in safety factor to keep from tearing the shock apart or bottoming it out when you install it at an angle.

When I talked to Aldan about shock mounting, they suggested 55% in bump and 45% in droop. I had shocks with a 4" range, so multiplying 4 times .55 told me to install them at 2.2" extended from their mid-travel point, leaving 1.8" for droop. I don't think it makes a lot of difference if you use 50% or 55% or whatever, just as long as you have your bump stops set correctly to keep from destroying the shocks.