1. Close the calipers all the way with whatever pressure you want to call "nominal"
2. Set the Zero at that point.
3. Open and close the calipers several times to to that same "Zero" point, and get the feel of the repeatable pressure that gets you that same "0.000" again and again.
5. Lock that amount of pressure in your "organic memory" and use it from them on.
6. Don't sweat it.