That's really clever!With a little bit of math you can discover all the different aspects about the shot.
The 32 ECT means 32lbs of pressure per square inch to crush the the edge of the box. Thus it holds no value. Boxes also have a burst strength (Mullen), and I think that's a proper value to work with as a starting point. For #35 ECT cardboard that's 200 PSI.
Since you know know your shot size, all you need is your shot weight. Along with the depth of each shot, you are on your way to filling in some physics formulas. Get ready for the metric system!