The way I've always done it is using the old pinion shim as a starting point, I install the ring gear/diff, set backlash, and check the pattern on both the drive side and coast side, and inspecting the pattern tells me if I need a thicker or thinner pinion shim. When doing all this I use an old pinion bearing honed out a little so that it can just slide on and off and easily change the shim, and I use an old crush sleeve and pinion nut. Once I finally get everything right and have a good pattern then I install a new pinion bearing, crush sleeve, and pinion nut and put it all back together and do a final check on the pattern.