VAT Calculation based on Formula

Hi

We have a requirement to calculate the VAT based on the following formula

1. If Sales = X, Surcharge = Y%, VAT = Z%
2. VAT = (X*Z%) / {(1- (Y%*Z*)}

I tried calculating VAT based on the amount 1000 , and came with some percentages for vA and created condition records for the same, the VAT is coming to 6.043%. The customer wants the VAT to be 6.04285 , which SAP doesn't support and requested whether VAT can be calculated as above.

I have never done this configuration before, I have an idea that the VAT can be calculated in this way using routine.

Can any one please throw some light on routines ? Or is there any simpler way to config this.

Thanks,

Vaishnavi