I've made 30 or so of these by hand like this. It really is muscle memory at this point. Without any ruler or measuring device I can make them the same exact size and shape every time, my wife will be watching me make them, she shakes her head and says "I don't know how you make it look so easy?" I've always said if you want something bad enough, you will find a way to make it happen. If you don't get what you want, then you didn't want it bad enough!
It's the same with the Moire rings. I lathe them out without picking up a micrometer. Each one is the exact same every time. Muscle memory!!