For most any purpose, as muscle development or strengh/power, going a little below parallel (90 degree break) is fine. The myth of going ass to the grass is never needed. Front squats will usually go just below that parallel point also Though Olympic lifters, when doing squat cleans, pretty much go ass tp the floor with very heavy weight. The guy in the photo looks like an Olympic lifter.
Of course there are a few opposing recent views, that a break around or below knee level is dangerious to the knees because too much stress is placed on the knee joints/tendons themselves. That stopping point (when going down) and sudden reverse of direction (going back up) is the blame. Personal view (for what it's worth) is that this is not the case for most people. The mechanics of the squat does not always support that theory for most men. Good luck.
