Your upside down pot, with the air that is trapped within it, can act as a drum. To judge the floor's thickness, tap the bottom of the pot. If you hear a sharp click, then the floor is still quite thick. When the pot sounds hollow when tapped, it has been trimmed enough.
Once the pot has been trimmed, release it from the bat or wheel. Smooth the clay surface with a slightly damp finger or sponge. Sign the pot, if you so desire.