If I understand you, your Work Center can hold 1000 items at the same time, but you do not use full capacity and sometimes use for 120, sometimes fo 200 items etc. And your WC always use 2 hours?
If this is a true, only way (without customization) is you do not use Run Time in Routing. You must make new PO for each oven and in Routing, you need to set e.g. 2 hours on Wait Time. If you configure that, each PO (120, 200, 300, ... 1000) will need 2 hours.
But you can make some minor customizations and use Run Time and Lot Size for it.
