Some folks have a tendency to cut a shoulder on the back plate that "precisely" fits the recess in the back of the chuck. Other than to complicate matters, I've never seen a good reason for doing that. Yes, I put a shoulder on the plate but, it's not so tight that you can't wiggle the chuck around a little. Once the chuck is positioned and the bolt tightened, it ain't going anywhere.
My procedure is like I mentioned below. Put a good shaft in the chuck and put a DI 1" from the jaws. Center the body on the plate until the RO is 0. Snug the bolts. Tightening them should not throw off the reading too much -if any at all. DI 3-4" further down the shaft. If the RO is within 3-5 thou, call it day. If the RO is more than that, you should probably grind and even-out the jaws. You can go through all this then, try a different diameter shaft -and chances are, it won't center as well as the 1st shaft did -and that's because of 2 things. A) The jaws float and never really lock-up exactly the same way. B) The scroll is probably not dead-on. If you get a $700 chuck, things should improve a little but, it's always a matter of time before something starts to wear out and things will get sloppy.
I happen to really like the chucks that came with my 1236. The 6" is nice and tight and holds well, so does the 8" and both only needed minor balancing. I need to fine-tune the 6" a little bit more but, I'm not losing sleep over it.
Ray