Formula for threads

I'm a self taught Lathe operator. I want to remove just enough from the rear of a 24 pitch thread to turn it about 80 degrees (really about 85 degrees). How much would my cut be? I know there is a formula for this but I can't find it. I don't mind making 2 or even 3 cuts, but I don't want to do it all night long. Any help out there would make me a Happy Camper.

If by "24 pitch thread" you mean a 24TPI (threads per inch) thread, you need to divide 1 by 24 to get the total pitch per thread, then multiply that by your angle (in degrees) over 360 to get the additional fraction of a complete turn you want. *Disclaimer: I am not a machinist.*

Divide 1.000" by 24 to find how far the barrel advances in one turn. Divide that by 2 to find out how much it advances in half a turn. 4 for 1/4 turn 8 for 1/8th. Turn
Etc.Etc. you will be cutting the barrel shoulder back to advance the barrel into the frame and adjusting the rear face of the barrel to set BC gap.

Thank You that is very Helpful and I 'm pretty sure I know what to do now.

I find it easiest to just shave little amounts off of the barrel shoulder until you can screw the frame onto it and have it get hand tight with the front sight about 10 degrees before top dead center. That way you don't even need to remove the barrel from the lathe to get very close to proper fit.if your BC gap was good before, measurements from the barrel face to the shoulder will tell you how much needs to be removed from the face after the barrel is set back. It can be done in just one setup in the lathe.

since you are asking the question of barrel setback, I'd suggest stoning the difference off the barrel face vs putting a carbide to it. Not by hand mind you - unless you've got the rotation down below 30-40 RPM.
Don't forget the B/C gap (all-around)!

Got it taken care of Thank You all.