I think that you may be overthinking this here. Granted, I'm in the screenprinting business and not the retail apparel business. We typically only stock black t's because that's what we use the most of. I buy them about 2,000 at a time. When they come into the shop we size them and put them on the shelves in 12s just as they are sent. When an order comes in we go to the shelf, count out what we need and take them to the staging area. They all tend to get low at approximately the same time, then we order more.
I don't know what kind of volume you're working with but I think that I saw a number of 100 or so. At only a couple of hundred pieces why are you making inventory sheets at all? My guess is that your Time would best be spent doing something else...like finding more customers.