We have a commercial 20+ machine print farm (Taz5,6) running 24/7 and need to quickly calibrate our printers on the fly. We found trying to eyeball the Z-Axis on both sides of the steppers was not accurate enough to get flawless prints. So we created this little jig that attaches on each side of the printer's top in order to get a sub-mil accuracy for the Z-Axis on either side.
This jig is super simple to create, then zip tie it to the top of the printer (see photos). Then use either a measuring tape (we now use a steel ruler for even more accuracy) to measure the distance from the head bars to the top of the printer. We calibrate our machines every weekend, and this jig speeds up the process 100x and gives us 100% accuracy.
NOTES: Taz6 works the best with this jig using the inside slot. It's 'almost' perpendicular using a ruler in the inside slot - but we actually use the outside edge with a steel ruler and get a true perpendicular measurement from head rods to top.
Taz5: the middle slot doesn't work so great with the Taz 5 since the head rail is set back a little more than Taz6. In this case, just use the outside edge of the edge.
Hope it works for you to get more accurate prints.