Partial VAT Base setup

Hello Expert,

I have the following requirement to done in NAV 2009 SP1 ,

VAT 75 % of Total Value ,can any give me idea what are step to configure, normal configuration i know but this split case i have no idea.

For Example : Amt : 1000   VAT 5.5% , now tax can be calculated on 75% of 1000, means the amount should be 750 base amount

So my tax amount should come (750 * 5.5 )/100   = 41.25

but it coming INR 55 which is wrong.

can any one help me on this

Regards,

Shailesh