I would do a "search" on test pieces if the precession needed was tight and really mattered. Use a formula or just plain guess. Try it, measure the difference, move your lines in the right direction by an amount based on the error and try again. You will quickly get to the precision you need and you will also discover other factors that affect how close you can get to the desired result (how precisely can you position and clamp the piece being bent?). I imagine that even the press break itself can have an effect on the bends. The formulas are based on a perfect bend and a given machine may introduce small error. How tight is the thickness of your material controlled? etc.
It is going to come down to some level of trial and error anyway, so don't sweat the initial calculation.
Just how I'd approach the problem with my own equipment.