Textile stok inventory management EXCEL

We have a leftover stock selling operation from a textile factory. Every month leftover clothes from the factory are recorded into our EXCEL database. We only sell kidswear pajamas, and it comes in 7 sizes (90 - 150). These are left over stocks so the quantity per size or per design is very inconsistent. We sell these wholesale to our customers, but we have to figure out a way to distribute both the SIZE, and DESIGN in the most balanced way possible. Here's what we do now: Total leftover clothes for a month "X" = 1000.( in 10 designs, in 7 different sizes. ) we count the qty per "d" design, then do, "d"/"X" x 100, for every design and see the %distribution overall. We do this because customers don't want any one single design to be over 20% of the overall package. So this is quite straightforward. The problem that I need to solve, is doing the same thing with sizes. Customers get no more than 20% of One design per package, but sometimes they end up with disproportionate sizes per DESIGN. (ex: for 20pieces of batman clothes they get 1 size) I want to find an automated way "EXCEL" to be able to distribute the sizes properly, so that customers can get a balanced package of both SIZE and DESIGN. Thanks