The extra shim is way too think to actually use in a race tune. If you want to use it for break in, that's fine, but to get a race tune with it is difficult.

The extra shim is .40 mm and the shims that come in the motor are .55mm. You almost double the deck height when adding it. If you need to adjust I would put in the .40 and remove the .30 or .25 shim, but using .95 of head shims will not yield good power.

For most applications, the stock head shimming (.55mm) after break in will work just fine.