How can I create an IF =0 then order x amount of stock stock 14 weeks before calendar month

I have a table with starting stock in, one with demand and some information on lead times for products. What i need to do is create a formula to look in the "opening stock" table, if that =0 or less then calculate say for example 14 weeks prior to the calendar month, then in that calendar month in the below "open PO's" table insert, say for example 1000 units of stock. 

Can this be done? An example spreadsheet is below. Thanks 🙂