We took the calculation from Honda's CRF450R Owners Manual:

(Recorded Valve Clearance - Desired Valve Clearance) + Existing Shim Thickness = Shim Thickness Needed

..and made this nifty little clearance calculator. If you enter the existing valve clearance, the existing shim thickness, and the thickness of your new shim, it will tell you what the valve clearance will be as a result of your new shim. That way, you can do some "what-if" comparisons with different shim sizes very easily after you have your "knowns". Then you'll know exactly what sizes to buy!

(NOTE: This is based on the formula from the manual, and while all the numbers make sense when we did ours, and worked for us, we take no responsibility for any damage you do to your motor; basically after you put in a new shim, you should always double-check your clearances just to be safe.)

Recorded Valve Clearance (some small percentage of a millimeter, like .15, or .25, etc):
Existing Shim Size (example: 2.15, or 1.70; something OVER 1mm!):
New Shim you want to try (again, not .210; a shim is at LEAST a whole MM, so enter like 2.10):