add_fee Tax

Hi There,

I am using the add_fee function to apply a discount to the total payable based on a payment gateway :-

Total 73.97 (inc tax)
Tax (20%) – 12.33
Apply discount of 38% for payment method (20.89)
New Total – 53.08 (inc tax)
Tax (20%) – 12.33 but this should be the new tax value, as the taxable 20% on 53.08 should be 8.85.

Does the order review not take into consideration recalculation of tax after a fee is added? Or do I need to use another function to work out a new tax value based off the new order total?

Any information on this would be appreciated.

Cheers
Jamie