When crimping for revolvers, how do you decide how much crimp you need? I know I need it enough to stop bullet movement, but how much is that? I tend to look at factory ammo and try to make visually similar.
Get a factory round, and use it for setting you "baseline". By feel, not by eye...
I'm talking here about using the built in crimp shoulder of the seating die. Not talking about any other separate die.
With the seater die body backed out, and the seater stem backed well out of the die body, put the factory round in the shell holder and run the ram all the way up.
Then screw the seater die body down, until you can feel the die contact the case. Firm contact, hand tight, don't gorilla it. This is the crimp shoulder in the die firm up against the crimp of the factory round. Then snug down the die lock ring to keep the die set in place.
This is you base starting point for a full crimp.
Then from session to session, how do you make it the same?
You make it the same by using cases of uniform length. Yes, this means measuring, sorting and possibly trimming.
You set your die on a factory round, compare the brass you have to it's length, and you'll probably find cases shorter and possibly longer than the factory round is.
Cases longer than whay you set your die for will be crimped more (possibly too much) and cases shorter will be crimped less (and if too short, not crimped at all). You can either adjust the seater die as needed for each individual case as you load it (time consuming and not at all efficient),
OR you can sort and trim your brass to one uniform length, so they are all the same and will all be crimped the same amount. That's the best way.
Revolvers don't care about case length, provided its below max listed specs. Recommended "trim to length" is usually about 0.005" less than listed max length for most revolver rounds. Some auto pistol rounds are less.
Measure your fired brass and sort it by length. Anything shorter than the trim to length for your caliber should be separated and processed separately.
All the cases longer than the trim to length get trimmed to the trim to length, and you have a uniform case length, to adjust your crimp to.
Now load 10-12 rounds using the crimp setting you did on the factory round. Shoot 5, check the unfired round for bullet movement. If it has moved, you need to adjust for a bit more crimp. IF it hasn't, shoot 5 more with that same round in the cylinder (so it gets the recoil effect of all ten shots) and check it again. If it goes 10 rounds with no bullet movement, call it good an start loading ammo.
This method has worked for me for over 50 years, in guns from .38Spl through .45 Colt.
Bullet creep (crimp jump) is not a constant force. It varies with the recoil of the gun (and the load being shot). Heavy slugs have more inertia, light guns have more recoil. What works in an N frame might not in a J frame size guns.
Your gun, and your loads determine how much is "enough" and that is always a trial and error testing process, requiring shooting. Be prepared to adjust your crimp setting as needed, based on your actual shooting results.
There's a bit more to it, but that's the basics.
Good Luck!
I did not cover bullet seating and crimping in one single step. It can be done (no matter what some people say), its not difficult, just another series of adjustments, and I generally do it that way. IF you do it that way, be aware you probably will have to change your settings if you change the bullet you're using.
The key is uniformity.