Thx for all the replies but you've been wrong. I think I know how to do it.
The best thing I can do is to model the entire setup in 3D using processing - keeping all the dimensions... that is I've got to setup up a gun model that can move to set elecation and azimuth and then use dist function to get distance from the barrel breacket where the wires are attached to both the step motors
I know it's kind of retarded but it will work, so I am fcking happy and feel damn intelligent (while feeling kind of dumb for not being able to come up with a formula) 