You have to start from the first day of incubation. Eggs are laid one a day until they are all laid, but not incubated until the last one is laid so they all hatch the same day. Sounds like the clutch is complete, as most lay 12-14. I would start the count from the day you saw her sitting on them for a while. She won't sit there except to lay the eggs until they are all ready or else the incubation will start. If you know the exact day she laid the last egg, that would be day 1.

Thanks Mrmk. Are you a duck fancier? If so are you aware of a way to encourage nesting in a particular place, or am I destine to spend my limited free time searching for the nests in the brush. I have two different nest shelter built that are shunned.