First of all, when you are doing crystalline glazes, or any other type of glazes, the thickness is really important, but particularly for crystalline and other "art" glazes.
You can buy a Starrett 643JZ at MSC Industrial for 319.97 and there are 35 in stock. You will have to grind the shoe down to a size so that it sets on top of the glaze -- mine is 3/16" x 7/8" and Bill Boyd's is 1/2" x 1/4" so it is not absolutely set in stone what size the base of the shoe needs to be. Mine does not work well on the inside of bowls and Bill has fashioned his for that purpose.
You have to keep pressure on the needle while you read the thickness on the pot or tile. I had hoped to get a picture of this today but no one came into the shop to visit -- and press the button on the camera while I make the measurement. I will try to get a photo tomorrow. I do have a way to let the top "thingy" that you press down rest in the middle knuckle of my index finger so that it keeps the pressure on naturally.
You also need to get a reading just after the glaze is dry enough so that it won't come off. You can wait 5 minutes but if you wait an hour, the reading will be off because the glaze will have dried, and it will be thinner.
This tool is nothing like a hydrometer. If I understand correctly, a hydrometer is a tool that you put into a solution and it measures the specific gravity. The problem is that glazes are not solutions, they are mixtures of chemicals and water, and maybe electrolytes too. So while the hydrometer can be helpful in that you can, if you use no electrolytes, measure the amount of water to the amount of chemicals and get that right every time, what if your water changes? Say all of a sudden your well water has lots of sodium. Your hydrometer may not give you the answer you want.
Most glazes that are used by crystalline potters need to be adjusted with either CMC, a flocculant, or both. The hydrometer does not seem like a good tool for this purpose.
There is another way to measure the amount of glaze on a pot and it is used by industry a lot. They just measure the surface area of a pot and put on a specified amount of glaze, so you might see the amount of glaze expressed in grams/square inch. My friend Phil Hamling, uses this method and you can see some of his efforts Here.
If Burlon Craig could read the glaze thickness from a shard, and I'm sure you could get that good, he would also have to make the interpolation from a wet glaze to a dry one. The shard is dry and the glazes are wet. The early folk potters were really in tune with their materials.
My friend Darrell Adams, who is the grandson of Bill Gordy, told me that he used to go out to find materials with his uncle DX and DX would tell him about the spirits in the rocks. I bet DX did not have a glaze micrometer.
However, that doesn't mean that I don't need one. It's been a very powerful tool for me.