Gift voucher and VAT

I am trying to get gift vouchers to work properly, but it seems there is a problem to how it does calculate the total price:

Example:
Order Total: 449.00
Shipping: 0.00
25% VAT 89.00
GV: -300.00
Total 208.20

Obviously this is wrong - the total should be 149,00

From what i can manage to figure out, how this is being calculated is as follows:
449 - VAT = 359,20
Then it applies the GV witht -300:
359,20 - 300 = 59,20

Then it somehow magically calculates the correct amount (149,00) but then goes ahead and add on the 59.20 to that amount.

149+59,20 = 208,20

Anyone who has any idea on how to solve this?

Thanks in advance - Trond C.