an inventoried products table, inventory transaction tables, and an ordered items table. May be a good starting point since managing the inventory replenishment will be a be a likely next step.
For this I would add another table related to the line item that tracks item qty and use that in the popover. This would allow easy update of how many feet used per job. If you enter total feet when you buy you can simply subtract the feet per job and track what is left. Also lets you track damaged material as well.