Prepayment is not calculating on TAX

Hi,

I have a Order worth $4000/ and Tax is 7% total order value is 4000+280 = 4280/-
I have set prepayment % to 20. So order value is 800 which is calculating perfectly but not the tax 56.
I would like to know will tax is going to be calculate in prepayment functionality or not.
I am using W1 nav 2013r2 build 13.

Thanks
GK