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If you are very worried about it then just sit the egg on the moist paper towel, rather than draping the paper towel over the egg. That way you will not be covering any more of the egg than you need to (apart from the bit that is already sitting on the ground anyway).
When the duckling comes out, just leave it in the incubator until the duckling is dry. Let it rest. It will be exhausted.
I know that you are really concerned to do everything that you can to help, and that is wonderful, but don't stress too much.
Keep in mind that mother ducks hatch ducklings all the time in all sorts of places and in all sorts of weather, so don't worry that humidity that is a little higher or a little lower than the ideal is going to be a 'make or break'.
Here's hoping for a good result!