Payables VAT/GST - calculate and add to total

Hi,

I'm looking for a way to calculate VAT/GST on a voucher when it is loaded through the Voucher Build process.  For example, the source system requests a voucher for $100.00.  I want Voucher Build to calculate that GST is due (10% = $10) and increases the voucher total to be $110.

Inclusive will calculation will work out that if the total is $100, GST is $9.09.  Exclusive will calculate what the total SHOULD be, but it just reports an error, rather than increasing the total.

Is this possible?

Thanks!