+1 on the over-misting. I thought there was no way you could over-mist your tank. Wrong. Last dry start I did, I misted 3X a day and started getting little white fungus strings all over my HC.
Misting frequency depends on the strength of your light and its height above the tank. The goal is humidity. I would just aim to keep your aquarium walls covered with a light fog of evaporated water. For my 20 gallon, I thoroughly mist my tank 2 times a day, while ensuring every plant looks wet. I lift the saran wrap when I do this. It's maintained a nice equilibrium the last few days--the water level has remained constant.
Regarding gas exchange, just leave a corner flap of the saran wrap open--this should be sufficient. Lifting the saran wrap when I mist also helps.
Sorry for the long-winded response. But I figure the more info/detail the better.
Edit: Don't mean to bogart the thread, but since we're dry starting and don't have water to reflect away light, could you use a lower wattage lamp over the tank when you dry start? I'd like to limit my energy consumption during the winter... 36 W over a 17 gal (60 L) tank?