You really don't know until you hook up. You can do a little math and get somewhere close. Or do what most of us did, and guess at it for the first hookup. Every truck and tt setup is different.
And yes, you will probably be moving bolts and changing angle.
One HUGE thing to do - BE SURE you have the ball tightened up. And by tight, I mean somewhere in the neighborhood of 450ftlb. In other words, a 200# man, using at least a 2' lever (wrench with a pipe on it) and basically sitting on the wrench. That will give you a minimum of 400ftlb.
If you don't get it tight, then you run the possibility of the ball loosening up on a trip. And that's bad.
And don't forget to grease both the ball and the bar ends.