Milestone billing problem

Hi everyone

I m in a complex situation where my client want billing milestone of two material quantity

total value of material is 100

milestone

10%

70%

20%

Now first bill when i create for 10% for one qty system picks the value of two quantity which i mention in order billing plan instead of one.

Now i want system should take milestone value based on quantity now complete milstone value copy from order.

Please help me in this.....points will be awarded

Regards

Shambhu Sarkar