Quantity based discounts in trade agreement


If i want to provide line discounts based on the quantiies, how it is possible in ax 2009

scenario 1 - If im buying 100 pieces, i have a line discount of 10 % and when im buying 300 pieces and more i should have line discount of 20 %

How can i specify this in trade agreements (ax 2009). I have tried this in trade agreements by creating two lines with different quantities and different discounts as mentioned earlier.But when i purchese 500 quantity it is taking as the sum of these two line discounts ie 20 % (10 and 20 %) which is not agreeable for me, i need it to 20 % only.

Thank you in advance for the solution.please reply soon