Have you cut a test piece in the chuck across 6"? You take a cut at the chuck end and another at the 6" point and then compare them. I am not quite sure what you are measuring?
You could also use a parallel bar to do the comparison.
I've not even bothered to take a few cuts with it. If I clamp something in the 3 jaw I can see how off centre it is with the naked eye. I need to get something to run true at the front end before bothering with the rest of the lathe.
It will be running true when clamped up, just don't remove job until finished machining. How do you think independent 4 jaws work.
I am on my 3rd personally worn out chuck, hardly a problem for items machined within a thou.
It just means you wont be able repeat putting in machined items square, but you wouldn't be able to do that any way with a new quality chuck to any great accuracy.
Better off buying soft jaws and machining them up.