Your losses won't mirror those of another, differently-situated issuer. I think the best approach is to review the losses recorded over the past couple of years (minimum) and see what percentage they are of debit card volume.
If you're new at the debit card game, perhaps some suggestions from those with more battle wounds could give you a starting point, but I'd recommend doing a look-back every couple of years to see whether your guesstimates (or forecasts, if you prefer) are reasonably good, and make adjustments as needed.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8