I read Roger M ... like this:
Backlash = play
# If too much play, recipe for disaster as the gear's teeth "attack" each other at wrong angle... skimmed gear
# If no play at all, desaster also because force=stress will lead to overheating and finally destroy the teeth
So shimming is fine art... As Roger puts it "feel the play with the sensitivity of your fingers, but so little that you can't see it"