Give my recipe a try. It is very fast drying and I rarely experience craters. It is also great when you are using light and dark shades together as it does not bleed. And it takes only a couple mins to make from start to finish.
The recipe is under Sugardeaux Quick Dry Royal Icing in the icing forum here.
I can COMPLETELY vouch for Karen's RI recipe!! It's the only one I use now. I don't have a dehumidifier, but I do put my cookies in my warm oven to help the icing set. It works like a charm! Just keep an eye on your temperature to make sure it's not too high.