I can still see light between the spindle plate and the chuck back plate. although the crack seems smaller. A piece of paper 0.0035 thick will go clear in to the taper in one spot and opposite that spot the paper will not start into the groove between the two plates.
rock_breaker, I am working through a similar situation on a new chuck, and the advice in this thread has been very helpful. My situation is very similar to what you describe, except I've confirmed that both the chuck and spindle face mating surfaces are very flat.
It finally started to make sense to me after looking closely at the middle picture in post #25 by JimDawson, where it points out the mating surfaces.
The taper centers the chuck on the spindle, and the large flat face ensures that the chuck is not installed at an angle.
In my case, I'm nearly certain that the bore where the spindle taper seats into the chuck is very slightly undersized. The taper seats before the flat spindle face is in full contact with the back of the chuck. I could fit a 0.005" feeler gauge between the two, but only on one side. The back of my chuck and the spindle face are both very flat. (Better than I can measure with a 0.001" indicator, certainly better than 0.005".) The location of the gap moved depending on which cam I tightened first. If I understand correctly, I am able to pull it out of square with the camlocks, since it is such a short taper and the flat spindle face is not in proper contact with the chuck.
I am working on the procedure Bob Korves described in post #26 to fit the chuck to the spindle taper. I'm currently on the "try again, and again, and again, little by little" step, but I am seeing definite slow improvement as I test fit and lightly sand the high spots. (I'm using finer sandpaper than Bob recommended, since I'm new at this and really don't want to overshoot. I only had time to mark and sand a few times last night, but now I'm down to a 0.004" feeler gauge.)
I wanted to share what I think I've learned, in case it helps. (If I've misunderstood, someone please correct me.)