The eventual goal is to have these for all my tools so I can preset tool lengths and never swap ER collets. This one was machined on a kind of sketchy lathe at work, so that's my excuse for the .0005" runout. In the future I'll bore them out in the mill spindle.
Technically the tool length depends on how tightly you fasten the collet nut, so I did some "blind" testing, tightening the nut without looking at the dial indicator. Seems pretty repeatable: