Yes, the procedure you've said is good. Don't force it when you go all the way to the end in either direction...you can feel it stop when there is no more room. When you're going back the other direction count the rotations as carefully and exactly as possible. Once you've done that, you'll know how many turns are in your box. Divide this by two, turn back to that point (from the end of travel) and and you can mark it for center.
Good luck!
