Steve,
From what I have read and seen, the Taylor shims are marked with a "value", i.e. a number. I don't know if it is an actual thickness measurement, because they are also tapered along the length. Taylor wants the two shim values to always have the same value difference between them, which, if I recall correctly, is 6. Thus, if the guitar comes with a set that is a Number 4 and a Number 10, replacements should be a Number 5 and Number 11, or Number 3 and Number 9, or something to that effect.
The point is they are a set. So if you remove .020 inch from one you will likely need to increase the thickness of the other.