Define Amount Left from one Layout to Another

[gharkins]

This may be me being dumb, but here goes!

I need to be able to calculate the amount of a particular item left, using a layout of amount sold today, and a product info page. The product info page has the current stock on, and the amount sold will be an edit box for any numerical input on the amount sold layout. I would then like to subtract the amount sold from the stock, and replace the current stock (to update). I am using FMP7 on MAC, and all ideas on scripts would be very helpfull.

Thanks

George