the shims come from the factory in 50 different sizes because they would rather there be a bunch of different heights available so the valve jobs can be half assed and cut way down into the seats. not that it has to be super precise.

as long as you are within .001" of cross height differential you are fine. according to nissan spec.

Also: you do NOT need to re shim the head when replacing valve springs. the only thing that will affect the installed height of the shims is if you have done a valve job. Changing the springs alone or retainers or even cams has nothing to do with it, so don't bother.