2 questions about form

The first is about box squats. I recently heard box squats are a good exercise to improve a lot of things, really (regular squat, deadlift, vertical) from a westside barbell article. I'm gonna incorporate these into my workout. I can already go pretty low squatting, so should I just start off with a low box?

The second is regarding good mornings. You're supposed to go parallel or lower, but I can probably get to around 20 degrees before I start to feel it. Does it matter? I'm gonna try stretching before I go GMs, but is it possible that my body gets used to it if I force myself to go lower?

Any other tips on box squat or GMs will help, also. :D