Calculate Plan in Requisition worksheet

Hi...

I m using Nav2009 SP1. I have an item A with replenishment type as purchase with a inventory of 80 pcs on hand. 10 is in the PO. My safety stock is 100 and the max inventory i can hold is 1000 due to storage issues.

My requirement is that if my inventory falls below the safety stock system should suggest me the balance qty to be ordered to reach the max inventory. ie., in the above case

My Inventory on Hand + Expected Receipts = 80 + 10 = 90
Max inventory is 1000
To reach the max inventory i need to purchase 1000-90 = 910 in a single line.

I don't want the system to suggest cancellation of the PO qty which is 10 and create a new PO for 920 or reschedule my old POs. If i set the planning flexibility to None in the PO, Navision simply ignores the 10 outstanding PO qty and suggests me to order 920 pieces which is not the required case for me.

I tried various options but some how the system is suggesting to either cancel my purcahse order for 10 PCS and recreate a PO for 920 pcs which is practically not possible.

Anybody got a good idea how to handle this situation.

Regards,
Venkatesh