Ass to the grass isn't a viable option as it was my inflexibity that caused the injury in the first place. I normally go just below parallel and even that requires a board under the heels. On the hack squat, however, I can go hams to calves for some reason.
Combine inflexible hams and hip flexors with a tall structure and you'll see why it is hard to go ass to grass without rounding the lower back. I do stretch, of course, but it helps only so much.If I didn't use a board under the heels I'd lean forward and I don't want that. I want to squat with an upright posture so it's mostly quads.