That is what I do. I keep the substrate as dry as possible. I learned from Sean at EbN, a little trick about mixing substrate. It's called the squeeze test. Mix your perlite and vermiculite (I use 2:3 mixture), and add a small amount of water. Mix the water in well, and grab a handful of substrate. Squeeze the substrate as tight as you can. No water should come out when you squeeze it, however, the substrate should form a solid mass in your hand. The solid mass should break apart very easily if tapped with your finger. If your substrate acts like I wrote above, then it is perfect for incubating.
With all of that said, if you notice your eggs dessicating later on in incubation, you can always add a bit of water to the corner of the egg box, and it will sink under the substrae, and help to raise the humidity in the box, and you will see your eggs plump back up.
I hope that helps,