How I earn my bread.
This is how I create the facing of every mouthpiece. If I am very lucky and do it right the first time, I can finish a Tenor in about 50 minutes - Altos take about 30 minutes. There are many crucial points when facing a mouthpiece. Placement of the index finger, the amount of pressure, the rate of the drawback, and the angle of the piece all affect the creation of the facing. Many, many times you will just need a little bit more taken off in one place and go just a tad too much and that's it because you can only take off material, you cannot put it back once gone. I use 800,1000,and 1200 grit silicon carbide sandpaper for the final facing.
After each stroke (and I do mean EACH STROKE) you have to measure your progress. Here you see me creating the perfect level to get an accurate measurement. Before I added the little aluminum bar for level, my readings were all over the board...
Then you use a set of feeler gauges to assess the facing curve. There are no rules to facing curves. I had to find what works best for me thru trial and error. Although there are no rules, there are established general guides - side rails should be even, facing should be smooth arc for even response (however some put "bumps" and "flatspots" in specific areas to give accent or diminished response in certain registers), flat tables, and a tip rail that matches the tip of the reed. Those are just a few of the myriad of factors to mouthpieces.