I have a 1 gal glass jar that I made into a terrarium in late 2005/early 2006. I completely skipped the charcoal (though this was my first terr. and not an especially good or researched one, given that I used sedum and hen and chicks in a completely enclosed terr. w/ English ivy. Needless to say, the succulents didn't do very well, except at dying), and have continued to skip it in my 2 terr.'s since then. I instead just used a little less than an inch of small stones, put soil on top of that, and planted the plants.
One thing I quickly learned was that you DO NOT need to add much water. I found that the moisture already in the soil is close to enough. What I would probably do would be to add a little water and see if it's the right amount, and adjust from there.
I recall that if you do want to use charcoal, you use a half inch of it and then an inch or so of rocks (?), though I forget which goes on top. You can also use a false bottom (I made one once, but used plastic canvas instead of egg crate, like this article mentions).
-Jmcat

Here is a link that might be useful: False Bottom Terrariums