Theoretically, you SHOULD be able to calculate what your fedback signal will sound like, but there are, imo, just too many variables to really do it. Phase, reflections, harmonics, time and frequency response of each element in the system etc., etc., just make this a formidable task. But, if you listen to Jimi Hendrix ( among other, lesser, players) you'll hear that he has quite good control of what he's producing. But that is where Art begins and Technology moves aside.