Cumulative values

Hi Experts,

I have a requirement to have Scales for cumulative values of a condition type.

Ex. If the customer buys 100 MT in the month of March he is eligible for Rs. 5 Per MT discount

If the customer buys 200 MT in the month of March he is eligible for Rs. 7 Per MT discount

If the customer buys 300 MT in the month of March he is eligible for Rs. 10 Per MT discount

as our normal scales will work for individual sales orders.. but i want to apply above said scale for the whole cumulative quantity in a given period of time.

Thanks in advance

Sree