iExpense VAT Tax Calculation

We have a business requirement where 20% VAT be applied to a receipt.  However, the entire receipt is not VAT applicable.  iExpense is limited to defaulting a tax code for each expense type and having payables calculate the tax amount based on code selected.  We need the ability to override the tax amount in iExpense or allow the user to enter a manual tax amount.  How does the integration with eBTax allow this.  If it is not possible, how have you accommodate this partial applicability in iExpense?

Thanks!