Technically, a box squat can be done using any bar position. They are most commonly done with low bar because people saw big equipped powerlifters do them.
There is also no correct amount of sit back required. The way you do them completely depends on why you're doing them, but some of the finer details of that is beyond the scope of my rushed reply.
People often through box squats into a programme without really knowing why. Their use is actually extremely limited, but they've remained popular mostly as a legacy of regular gym goers copying equipped powerlifters, which is stupid.
There are some basic rules though:
1) You do not rock back on to the box. Doing this you are moving around mobility problems, and in fact it's probably those exact end-ranges you want to be training.
2) You do not rock forwards off the box. This is simply cheating as it artificially loads the posterior chain, which has no functional carryover to your normal squat.
3) Your box must put your hip crease below your knee. There are a few exceptions to this, but in general this is how they should be done.
