mile stone billing

iam using milestone billing

i want to bill the customer based on the quantity but on percentages defined in the billing plans

how can i do this

billing is based on the milestone dates defined in th billing plan

but when we do billing the quantity in the billing is taking as the whole order quantity

but i want to change the quantity according to my reqmt that is i want to bill the customer based on the quanitity not on percnetages

plz guide me on this