Re: Squat Depth
Past a certain point my butt rounds because my hamstrings are holding my knees back. Increasing the angle so that my legs are tucked more into my torso results in a rounded back. Everything has to relax to get the angle. Instead of getting a more acute angle so that the torso and leg are tucked in the shins take some of the slack, so the back doesn't have to shift anymore and the hamstrings don't pull so much.
Try for full depth but for you, if the rounding occurs and the knees slip forward, you'll lose all your power springing back up, which is very bad. Test out your depth with various box heights. Alternatively lie flat on your back and bring your knees to your shoulders, noting exactly where your hips come away from the floor in order to force the angle.
I hope I just made sense... at least that's what I gather from your post.