Tax Caluation only for partial quantity in sales order

Hi Guru's,

Please provide the solution for below scenario in EBtax, One of our client required below test case to calculate tax.

Sales Order Quantity --> 5

Enable DFF in Order line level enter Quantity --> 3

Then Tax should be calculate for 2 quantity only.

Line Value --> 100

Tax %  --> 5

DFF Line Quantity --> 3

then Tax calculation on below manner.

Line Amount= 100*5 ---> 500

Tax Value = 100*2 (5-3) --> 200*5% =10

Total Value = 510.

Please any one guide us how to achieve this requirement.

Thnx & Regards,

Udhayan N