Your method is not a very accurate one. Tightening the collet pulls the cutter UP, using cutters that are exactly the same diameter (0.0001) and using a torque wrench will help.
Best method: Reset your Z for each cutter (using the same spot). For wood, I use a 1mm metal shim (so I get a more consistent measurement, due to the wood’s irregular and soft surface.)
But usually, for a sign…Z height isn’t that critical. (+/- 0.004)