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Silly question, but I've had people tell me different ways to check the clearances. So thought I'd pole this for your opinions.

When inserting the feeler gauge is the correct measurement for the gap either:
1. when the gauge is inserted freely between the cam & the shim and can be moved back and forth smoothly or-
2. when the next size gauge will not fit, take the last measurement (although it grinds when fitting through the gap).



The blade should fit smoothly, and almost glide in, without being forced in, or feeling like it's getting stuck as you draw it out. Your instincts will tell you when it's right.
Feeler gages aren't ultra precise, but they're good enough when a 4 or 5 thousandth's tolerance is indicated.

FWIW - my 05 Speedmaster with 4,000 miles on it is still well within tolerances, even with an SAE feeler gage.