It's also important in glazing crystalline pots that the glaze thickness be checked as the glazes are very sensitive to how thickly they are applied. My answer to this problem was to take a Starrett JZ 643 Dial Depth Gauge and modify it so that it works as a measuring tool for checking the thickness of a glaze. The base of the shoe is 7/8ths of an inch by 3/16ths of an inch. This gauge will measure the thickness of a glaze to the nearest half thousandth of an inch.
To use the meter you press the button on the top and a needle like point comes out and you stick that point through the glaze until it hits the bisque and then, while still maintaining pressure on the point, you allow the bottom of the shoe to rest on top of the surface of the glaze.
This is one of my most valuable tools. I would not glaze without it.