Mag Length
The answer sort of depends on your mag.
I usually set the length to equal that of the factory round I'm loading. If you're doing something beyond that, like loading a cast 255 SWC bullet intended for 45 Colt, for example, here's what you can do.
Some mags are exceptionally long - others rather short. If you load to industry standard length, you should be just fine. To see how long your mag might allow, take a factory round, put it in the mag, and push it down with your thumb so it's as deep in the mag as the 'second round' would be. Observe the space between the front of the mag and the front of the loaded round. That gap is how much room you have to play with, more or less.
Mind you, that's just an eyeball check and you still have to work with variables like having enough bullet in the casing to allow for a proper crimp, pressure generated, and all that.