Hi. Allocations and Pricing using Sales Agreements

We are implementing Sales Agreements and require the ability to Price separately a Fixed Agreement and a Flexible Agreement based on a Weekly Allocation.
We are finding that the Sales Agreement used switches to the flexible agreement as soon as the order quantity exceeds the remaining Allocation qty.

Is there a way that we can set up the Agreement Quantity Detail so the last order that consumes the Fixed Agreement picks up the Fixed Agreement and the next order the Flexible agreement

E.g. Can a tolerance be set using the Quantity MIN and MAX fields
Regards