Piston to bore clearance is always measured as a total reading, the difference between the bore diameter and the piston diameter, Then apply whatever limits you use...Zenoah's limits are .03-.06 smaller than the bore for a new piston, and if the piston is .15 smaller than the bore it's time to replace...I suspect Echo and others use the about the same limits...
No one would ever replace a cylinder when the chrome is still good, regardless of whatever miniscule wear could be measured...by the time any wear is detected the chrome is gone..
Cast iron cylinders are another story, they wear out at different places in the bore...