While I'm new to leather working. Quickbooks has a pretty good inventory tracking system. If you are looking for a bona fide all inclusive business solution software checkout Quickbooks Enterprise Accountant. Unfortunately it's not the cheapest in the world at $70/month. But cheaper than hiring someone. Quickbooks allows you to enter an item which covers how much leather you use within a certain project, as well as other costs such as hardware, dies, ect. The benifit is as long as you use fairly the same amount of leather per product, you only need to change the quantity that you have of your finished product and your costs and inventory are automatically tracked.
Hope this helps!
Sam