Ok, I assume a 2.5' trailing arm length, which becomes the radius of the arc (the circle) that the axle makes as it is moved down to lift the rear with the gen2 springs. This means a circle of diameter 5' x pi for the circumference of about 15.7', or 187", divided by 360degrees in a circle equals about 1/2" per degree, very roughly. If the axle centerline is 5.5" from the trailing arm bolt hole, that's a 11" circle, or a circumference of 36ish ", divided by 360*, or about .1" per degree. You are out about 2*, so .2" hole movement.
Your math is correct...but I think the measurement points need to be different.
Making a couple of assumptions:
-The output of the T-case is a fixed angle based on how it's mounted.
-The angle of the trailing arms (in a neutral position) is fixed due to the weight of the truck (and Larry)
Basically the axle needs to be rotated within the trailing arms, and the length or angle of the trailing arms doesn't affect the math at this point.
So, really the measurement depends on the distance from the trailing arm "pivot" bolt on the axle, to the pinion flange, and the distance of the slotted "adjustment" bolt to the "pivot" bolt. After that you should be able to calculate it the same way.
A way to visualize would be like a step ladder...The distance at the feet is greater than at the hinge, but the angle at the top is the same. The hinge is the adjustment bolt, the feet are the pinion. A small movement at the hinge results in a big movement at the feet, but they still have the same angle.
