I calculate my shrink at the lowest possible sale price, which is usually 33 cents. I run mostly mint boxes, so with only one product is pretty simple to figure out. Although, I mostly concern myself with my actual profit. That's the number that means the most to me. If I have a box making me $30/mo. in profit but the shrink is say 40%, I'm going to be okay with it.
It's impossible to know exactly who took how many. You're best bet is to figure off of the lowest price point, since it's more likely that peel are taking the better deal.