The wavelength of a 10kHz sound is 3.4cm, which means that, assuming a single point source (monitor or PA) and single point mic, you go from a full positive feedback (perfectly in phase) to full negative feedback (perfect counter-phase) at that particular frequency simply by changing the distance between them by 1.7cm. Things get awfully more complicated with multiple sources, non-puntiform sources/mics, reverberant room, mic directional patterns, etc., and of course each frequency will dictate its own phase relationship. And then there's sound processing introducing phase shifting…
It's just impossible - in a real world situation - to calculate the exact mic placement that reduces the feedback, it's a lot more practical to use some good sense (closest to the instrument as possible, low-sensitivity direction pointed as much as possible toward the closest monitor) and then move around the mic until you find a good spot - if there is one at all.
(And of course it's misguided to think - in this context - that there is a "right" and "wrong" wire phase for speakers and mics.)
Once you placed your mics with some sense, the solution to the feedback problem must necessarily be reactive, not proactive: you hear the feedback, you lower the gain - possibly just at that frequency with a notch filter; where "you" could be a human or a piece of hardware or software.