I think all you can really do is get a second Dr.'s opinion. Nutha fitters fit/opinion.
I think I'd use a shorter crank arm before I'd get to 6mm of shim. But not claiming anything but guessing...
Seems though I read process was to only correct 50-60% of such discrepancy, not 100%. Or to do it in steps if a lot, 3mm, then 1000 mile later the other 3mm or something along these lines.
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