Inventory Problem for Coffee Roasting

I sell freshly roasted coffee beans and have hit a point in my process where my success using Squarespace's inventory feature simply is not sustainable. Here is my problem: I sell 12 oz, 1 lb, and 5 lb bags. It takes 1 lb of green coffee to roast the 12 oz option. It takes 2 lbs to roast for the 1 lb. It takes 6 lbs to roast for the 5 lb option. All of that coffee pulls from the same stock bag so if I have 100 lbs of green Guatemalan coffee, I can say that I have 100 bags of 12 oz, 50 bags of 1 lb, and 16 5 lb bags available. HOWEVER, if someone places an order for, let's say, a 5 lb bag, I have to manually go in and adjust my 12 oz and 1 lb bag inventories EVERY TIME someone places an order. Is there a way to have the inventories automatically adjust for the other size options?