Stock by Attributes - managing generic stock used to sell different products

I sell tshirts.

I have over 50 products, each product is a different design, but come from my stock of shirts- i print on demand from blank shirts I have on hand.

On almost each one you can select type ( long sleeve, normal, singlet) and size. All product ( shirts ) are printed from the same pile of stock i have - i dont put 5 shirts aside for this design, 10 for that etc...they are all together, and drawn upon as I sell a particular design.

I've installed the latest Stock By Atributes, but now it seems you need to set up the stock supply levels per product.

Really what I need is to apply a supply level of, for instance, 50 medium shirts to all products, and then when one product ( a shirt design ) is bought using this size, make all products show supply level of 49 for medium shirts.

I am happy to share my store URL is thats allowed/helps?

Does Stock by Attributes doe this, or do I need another mod? How would you approach this?

Many thanks!!! ( i am loosing my head counting shirts all the time!! )