"as I understand it, I can carefully mic the distance from the top of the cone to the axle tube to determine the shims I need. Is this right?"
Yep, that's how I do it anyway.

Once you've centered the assembly, fit one of the backing plates with roughly the shims it needs. Bolt it tight.
Then measure the distance between the red lines on the other side - 0.019.

Rather then measuring (with errors) I just trail and error it. Hold a straight object againts the cone/bearing shell. Then take the amount of shims you think you need and try to fit them in between the 'object' and the axle tube flange. It should be a tight fit, or better: the shim package should only just not fit.
Do this on both sides of the cone since it might be just a tiny bit off.
A bit vage, but hope you get the idea.



cheers, ben

ps. what do you think about my drawing skills?
--
Daily driver: P130, '65, B18+M47. In the proces of restoring: P131, '69, B20+AW71L. (www.knutselsmurf.web1000.com)