Wait, hang on.

You want to do the test on level ground -- you are correct. (I usually use a parking lot.) However the truck should be sitting on that ground "as it drives." If you jack up the truck when you do this procedure, you'll get an incorrect calibration.

What you're teaching the truck by doing a ZPC is "This is what the truck looks like when sitting on flat and level ground." To make an exaggerated point -- if you performed the ZPC when the truck had a flat tire in the rear left, the truck would then think that the front right end is dipping forward several degrees in that corner once the tire was repaired.