Originally Posted by Jeff Belyeu
I think I start with putting my hips back, and then lower with the same back angle, but then for the last six inches I guess, my butt comes in and my back goes more vertical. Should this depth only be for air squats, and/or high bar squats? Should low bar back squats stop X inch(es) below parallel, and high bar or air squats go into a third world squat? Thanks in advance.
On a back squat, you only need to break parallel a little. There's no need to go 3 or 4 inches below parallel, especially not at the expense of lumbar tightness or what you describe above.
If your hips are coming in at the bottom, that's not good. Your knees are likely coming forward too, and you're probably losing all the hamstring tension, and the drive up probably really lacks hip drive and becomes more a quad exercise.
Rippetoe's new Starting Strength DVD shows him correcting several people with similar and related faults. That may be money well spent for you.