Proper piston to wall clearance depends on the power level of the engine. Use the chart above as a guideline. Minimum clearances are fine for naturally aspirated street applications. However the clearance should be increased for higher output engines, especially with nitrous and forced induction (turbo and supercharged).
For the most accurate measurement place your micrometer at .500” from the bottom of the skirt 90 degrees from the pinhole.
Take a look at the Anatomy of a Probe Piston below to Find Out: