More musings on the optimal throat angle. This post assumes we are nose-sizing and loading the bullet Ardito style.
Generally the Ardito-style bullet is seated 0.100" - 0.150" deep, so let's set the full diameter portion of the bullet at 0.150" long.
You want the front band to engrave slightly, so let's set the front band diameter at groove + 0.002".
For a given bullet, you want the throat angle to line up with those two points on the bullet:
Here are some concrete examples:
6x45, 60 gr. spitzer, .243" x .237" barrel, top of front band 0.35" from base, matching throat angle = 0.9 degree per side
30 cal, 130 gr. spitzer, .308" x .300" barrel, top of front band 0.48" from base, matching throat angle = 0.7 degree per side
30 cal, 180 gr. spitzer, .308" x .300" barrel, top of front band 0.741" from base, matching throat angle = 0.39 degree per side
357 mag, 150 gr. spitzer, .356" x .348" barrel, top of front band 0.36" from base, matching throat angle = 1.1 degree per side.
A longer bullet will require a longer leade which will require a gentler throat angle.
In practice, there will be tolerances in machining the reamer, in cutting the chamber, and in cutting the nose die, so there is no point in worrying about getting the angle perfect. When in doubt err on the steep side because you can always cut a short throat longer but you can't cut a long throat shorter.
This is just a theory based on my recent experiences using the Ardito method of sizing and loading. Feel free to form your own conclusions.