Paul and Britton,
Thanks for your input, but I think I'm not communicating our needs very well. Customers buy in many different increments (like 1/3 yard, 7/8 yard, etc). So, it's impossible to calculate how many units I have available in inventory if I start breaking it down into all possibilities. I just want something that will subtract whatever unit is purchased from the total of 15 yards. I have over 800 bolts of fabric in the store and have had several issues of overselling on the website because someone bought the fabric remaining on the bolt in the physical store.
I looked at Buy the Measure, but they only calculate in whole units. So their suggestion was for me to only sell my fabric by the foot. That is just not a viable solution.