For a store that sells ammo I have set up quantities as attributes. They are sold in quantities of 50, 100, 250, 500 and 1000.

I would like to have the stock levels be counted towards a main inventory count. For example if I have 100,000 rounds of 9mm I would like the sale of a box of 1000 to come off the total available and show that there are 99,000 left. Whats the best way to do this without using the SBA module?