QB inventory system isnt complex enough to do what you hope.  Mostly QB just tracks buy/sell items. In desktop you can also create 'group' or 'assembly' type items, but neither of those matches your situation.  Presumably you cut the sheets into a wide array of things, and there is wastage. Although you discuss PSF price/cost, I doubt that area is the only thing that determines price.

Best I can suggest is that you can track the purchase of large panes as inventory items, and even separate them into a 'raw material stock' account. But the sales item(s) will have to be 'non-inventory' and you'll have to manually reduce the stock of full panes as they are consumed.