Are you making an actual claim for a refund? If so, the process would be quite different.
Working out the additional cost of not being on a tracker into the future is a bit more difficult. You should work out the Net Present Value. I suggest the following.
For example, the repayments on €100,000 @ 1.5% over 20 years are €482 per month.
The repayments on €76,000 @ 1.5% over 20 years are also, €482 per month.
So the cost in today's money is around €24,000.
It's a bit more complicated that that, but you are only making an estimate and that would be the rough ball park.
Brendan