What are you defining as the directrix? The frame or the ground?

If the frame is set parallel to the ground and the axle is moved up / down then the differential angle can be measured relative to gravity. Furthermore, the differential angle relative to the ground is irrelevant. The angle of the differential is relevant only in respect to the transmission.

It would seem that raising / lowering the axle, with the frame parallel to ground, and the angle measured relative to gravity would provide the more accurate result.