The taper of a novarossi sleeve is about 0.3 degrees with the length of 32mm
Over the 32mm there is a diameter change of 0.15mm, that is 5 micron per mm height.

With Novarossi it is almost ideal to have a pinch starting 6mm from the upper edge of the sleeve. From experience I can tell you that 1 mm more or less will result in a complete different break in process resulting in still a lot of pinch or no pinch. That is just +/- 5 micron.

Finding machines that have a tolerance within a few microns with a reasonable production time is hard and pricy so the litle time spend with fitting pistons in sleeves is pinuts.