Isn't the 2010 screenset more aimed at probing?
Regardless, if you know the size, touch of two adjacent sides, and work it out from there (half probe diameter + half width/length will give you centre zero)

Either that, or you have to touch of opposing sides, and calculate the centre. Simply take the position from the touch on opposing sides and average them out (add them together then divide by two - you don't need to worry about probe diameter as it cancels itself out when averaging).

If you've got lots to do, I'd be making a repeatable jig/stop so you don't have to re-centre for every box. It doesn't have to be fancy, just a couple physical stops so you can repeatably clamp the box in position.