Rebate Agreement & Scaling

Hi,

Customer Qualification for the Rebate is based on Sales Volume for the period not for the Sales Order / transaction e.g. Rebate Agreement Period is 1st Jan to 31st Dec. & if sales volume for the period reaches 40,000qty for that customer then whatever above that will be applicable for Rebate & for every additional 1000qty rebate % will be increasing e.g.

Start Date   - End Date    -  Sales Volume     - Rebate %

01-01-2014   31-12-2014     40,000qty                 0%

                                        41,000qty                 5%

                                        42,000qty                 5.5%

                                        43,000qty                 6%

                                        44,000qty                 6.5%

This scaling is same like "Graduated to Interval Scale" - scale type "D", which is possible to map for the particular transactional volume, but here I have to consider volume for a entire period.

How to map this scenario?

Regards,

Shantanu