Occurs to me from some of the posts here that basic feeler gauge technique might be useful.

For checking, we used to go plus or minus a thou, but that was with relatively wide tolerances.  On these tight set little engines, I'd want things pretty close.

The key on feeler gauge use are to first get the gauge in and under no particular twist or force, then to slide back and forth to feel how much drag there is.  The gauge can actually push open the valve a little bit, so you just want a nice smooth lubricated drag.  Not loose or easy and not particularly hard.  Get the same feel on all the valves and things will be much happier.