I have a similar question. In order to do targeted money-saving, I already bought some gift vouchers at Dragonbox. (but much lesser, would just be a part of the downpayment)
So although you already have that part of profit, it would may be complicated to include it in your calculation.
So should I also better wait to use them for the final payment?
That shouldn't be a big deal, you can use them anytime you want.