Ok, I’ll take a crack at it.
First, I’m assuming you’re mitering the seatstays free (ie, you haven’t welded them to dropouts yet) and that you’re done with any bending/manipulation you plan to do.
Step 1: Adjust your dropout dummy cones to approximately the width of your actual dropouts. You don’t need to be particularly careful about this, just get close. My fixture is old enough that it doesn’t even have a scale for this, I generally just eyeball things these days.
Step 2: Install seatstays into your clamps/v-blocks and make sure they’re even/centered and that they’re not twisted out of plane. This can be really quick if you have a newer fixture with a centering wedge, or it can take a little futzing around if yours is as old as mine. Regardless, though, having the stays perfectly centered in the fixture doesn’t really matter. Having them extending out from the clamps evenly is more important, and you can accomplish this with the aid of a parallel and your eyeballs.
Step 3: Set your desired angle (generated from a CAD program or via whatever method you use) and get your hole saw centered. The quick and easy way to do this is to back the table away from the spindle/saw a bit, lower the saw to the point where the teeth are below the stays (so that the side of the saw will touch the stays as you tram in), then start the machine and sneak the saw in closer and closer. Since hole saws are never perfectly round, you’ll get to the point where the high spot on the saw is just tapping one stay. Once you get it just barely tapping both (your eyes and your ears will tell you) you’re good.
Remember, you have to have the stays extending evenly from the v-blocks for this technique to work.
Step 4: In some cases, your setup might not be rigid enough to keep the stays centered, since the saw is pushing one stay and pulling the other. If you find that your miters end up slightly offcenter because of this, you’ll want to tram over on the X by a small amount (ie 25-50 thousandths or so) to compensate. Experience will tell you how much (keep notes) so that you can compensate on the initial cut going forward.
Step 5: Seatstays are thin and hard to hang onto! It’s easy to mess them up in a mill, so don’t fully tighten the drawbar. Just go hand tight, no wrench. If you do snag something or break a tooth, you’ll just spin the arbor in the collet instead of destroying the stay. And no, you won’t hurt the collet or the arbor doing this at bike mitering speeds/loads (unless you leave it running for a few hours). Don’t do it for anything else, though.
Step 6: Miter away.
Hope that makes sense.