I have been using a corner of an index card under the cutter. I jog down until it just grabs then I know the surface is 0.010" below... the card is 0.008" and the additional 0.002" is the gap that lets the card just move without tearing. (Verified by measuring a step machined in to a block.) I like using the index card because it gives a bit of a cushion should you over run, preventing the material from getting a dimple. It is also nice that a nice round number, 0.010" is the offset.
Since I installed the touch probe yesterday I will be moving towards using it and the known heights of each tool. This works for me since my tooling is being installed in TTS holders so they always go back in at the same height.