OK, I'll take a stab at this. I used to align gas turbine to generator couplings of over 200,000 HP as a young guy, and taught a few millwrights how to do it.
Ok, first thing, get the outboard end close first, that would be the muzzle end if you are chambering. I write with a Sharpie on the spider 1,2,3 and 4, 90 degrees apart and set the zero on the DTI on #1. Next spin it around to #3, and check the offset. If it is say .020, then move it .010, split the difference, roll back to #1 and it should be a lot closer. Repeat same deal for #2 and #4, all on the muzzle end. Move to breech end next.
By whatever means you like, range rods, extended DTI etc, do the same on the breech. Go back to the muzzle after you get close, not dead nuts but close, it will change when you move the opposite end. Go back and forth, between the two, obviously the breech end has larger effect if you are working the chambering operation.
If you are using range rods, then you have to NOT worry about muzzle runout. It takes only TWO points to define a line, three ( muzzle and taper/slant on the rod ) are three. Get the breech straight and square, and go forth.
FWIW, the best BR smith I know, zeros the muzzle, the neck of the breech and prebores the chamber. 2 1/2 World Championships in a row, in that game, is good enough for me. By zero though, I do mean zero, 2 tenths or less measured on the grooves.
Also FWIW, he also cut his baby teeth zeroing large rotating machinery....... 24" diameter couplings, driving tens and hundreds of thousands of shaft HP. This stuff is pretty simple, but it does take a bit of practice. If you make a move with the part, and the DTI shows a different, unexpected result, you did something wrong, or your setup is bogus.
Hope this helps, good luck.
