Summit sells a tool by Proform, that includes the dial indicator, the jigs, and the extensions, for about a $100. In their catalog and on their website.
I just finished doing one the hard way. I cut a piece of aluminum angle and made holes to bolt in on the differential cap bolt holes in the case, and a hole over the pinion head. Had the face of the angle surfaced at a machine shop that does heads, to make it flat. I used a dial caliper, the extension end, to measure through the hole to the pinion head, and subtracted the angle thickness.
That gives the axle/differential centerline to pinion head depth. Took four tries, pressing the inner pinion bearing on and off, but it is within .002 by my measurements.I put the stock differential shims back in, and have .008 in backlash.
The contact pattern looks ok, maybe a bit too toe oriented.
I am going to sleep on it and see if I think it is ok, or maybe subtract a couple thousanths of pinion shim and try again.
It helps that I have a homemade case spreader that bolts to the differential case bolt holes, so I do not have to hammer the differential shims back in. That would be the most difficult part to me, especially with the aftermarket stackable shim packs.
I will probably buy the tool soon, just because, after three of these, I want to do it a bit easier.