Here's what I use. These depth gauges are available as "tire tread" gauges for under $10. You can find somebody who will gladly sell you the same thing as an "official mouthpiece gauge" for $50-100 because they claim that it is an "official mouthpiece gauge." As you will see in a moment, the tip opening is the least important measurement on a mouthpiece, almost of no importance, but it continues to get a lot of attention. It is actually about as important, from a performance perspective, as the color of a car.
Most often, if I'm working from table to tip, I don't want to touch anything to the left of the marking in the picture below, as I've already gotten that perfect (actually, I usually get close and then go back over the whole piece with really fine paper). But I still need a way to keep track of my progress.
This Goldentone has a big problem with the rails being uneven from side-to-side. If I just place one rail on the sandpaper, I can work one side down, but that will actually put a "tipped" surface on that rail because that rail is "up" on the paper and other rail is "down" on the glass surface. It is a tiny difference, but the feeler gauges will only measure the highest side of a rail (here, to the outside of the rail) while the reed might actually sit towards the inside of the rail. Either way, here is one way to reduce the problem of canting the mouthpiece rail when removing material from just that rail.
Here's the new reworked curve compared to the computer generated Link 7# line. The lay could be improved, but remember, it could also get worse. When I'm this close, it's time for a play test and forget about graph points.
The final graph curve looks nice and smooth, because the graph program averages the measured points to artificially produce a smoother curve, but it's not ever going to be a true representation of the lay. Think about it. The lay could be a series of 12 flat planes, with each feeler gauge measuring the center of the plane, and the computer program would artificially smooth the number series to make it look like a perfect arc.
So there you have it. A "free" hot rodded Goldentone with a .111 tip, ported and polished. How does it play? Better than your expensive mouthpiece. No way, you say! Don't worry, mine still has one huge unforgivable flaw on the outside. It says Goldentone.