Key thing is, keep the shins vertical. This means you need to stick your butt out of course for balance. If you don't feel it in the posterior chain but in the quads chances are you are pushing the knees forward. This is asking for knee trouble. Have a look at The first rule of Squat Club | Thor Falk if you want, but the rules are simple: (1) shins vertical, (2) stay on your heels (this is a corollary of 1 of course), (3) keep your back straight, especially the lower back, (4) go down at least to parallel, but watch your back: wider stance, toes out if necessar; stretch your hammies
Loads of good material out there on the web!