Setting up requistion methods PURCHASING

I have the following issue for my requistion method: 1. Reorder points on SKU-card of the item have to be taken into account 2. Demands coming from sales orders have to be taken into account → So far no problem 3. If an item is reserved on a sales order, the reservation should be held on the purchase order (without having to manually reserve it on the purchase order!) in order to see the reservation on the sales order (and obviously keep the item reserved after goods inward). The only method the reservation part works with is with “Reordering Policy” = ORDER. BUT with this method the requistion worksheet always orders the exact qty of the sales order and does not take inventory into account. BTW: We are using 3.10A-GB. Any suggestions? Thanks.