How consistent is the ring on taper tooling?

I recently changed out the spindle on one of my small high speed
machines to an ISO20 quick change spindle. I'm trying to determine the
easiest way to measure tool length off the machine. As near as I can
tell the only purpose of the ring on ISO20 is to hang the tool holder in
a tool changer. To do that it really doesn't have to be all that
consistent.Well not as consistent as tool length measurement needs to
be. Do I have to use an actual tool zero, and machine or buy a taper
socket to set the tools in on the surface plate?

I'm not using a tool changer. Just doing quick changes and wanting to
use the tool table to reduce time doing touch offs. Just do it once at
the beginning of the job.

So far I really like the ISO20 spindle. Its already saving me a lot of
time over the ER spindle, and I have a second one ready to go in a
second machine when I have a spare afternoon to do it.