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@MxnWhoWalks You should be fine. When it comes to chains, it's basically just matching the number of speeds to what's on your bike, the measurements are all pretty standardized.
The 6.6 figure is likely just 6.57 rounded up. Even if it's not, compare that to the widths of 8-speeds (7.1mm) and 10-speeds (6.2mm). Comparing to the 9-speeds, that's more than a 10x larger gap than between the two chains you're looking at. The 0.03mm difference is nothing.