Sevo,
I think you have received some great info here, but I thought I would throw out how I do it. I use the same thing as you, a 2:1 Vermiculite/Perlite mixture. It worked great for me last year and I didn't have a problem with a single egg. I only lost two eggs, that were looking bad as soon as I put them in the box. As for water, I tried to measure evrything last year to come up with some kind of formula. I never could come up with a set recipe for incubation media ( I was trying to do this for my wife who was going to be setting up clutches for me while I was offshore). It didn't take long with trial and error to figure out the amount of water to add. I agrre with Adam, that after a few times, you'll get to where you can feel the medium and know if there is enough water or not.

Also, I do preheat my incubation box with the medium in it. I also keep 5 to 6 jugs of water in the incubator at all times. It helps with Heat exchange, i.e. stabilizing the temps quicker after opening and closing the incubator, and it also allows me to add water, that is the same temperature as incubation, to the incubation medium, if needed, during the entire incubation process. Hope that helps.