Calculating Remaining Stock

Hi,

I'm having trouble with an creating a measure to determine remaining stock left for a given product during a given time period.

I have:

SalesTable (order line level, with Style ID, Order Date and QTY Shipped)

StyleTable (Each Style ID with their initial stock allocation)

Dim Date

I would like to be able to show on a bar graph, how much stock is left for a given product.

I figure I need to create a measure which shows the number of Sales to Date, one that shows total allocation, and a third measure which subtracts the Sales to Date from the Allocation.

Any help would be immensely appreciated.