Brendan, the reality is that noone knows what is going to happen to the exchange rates in the future. Detailed and carefully thought-out predictions can be made - but they are only that - predictions. Events can always intervene in unexpected ways to cause big short or long-term swings.
If you want certainty in what you will pay, it should be possible for a bank to guarantee you a fixed exchange rate at a specified date in the future. They will charge a fee for this - but you have the benefit of knowing exactly how much you need to pay.