I've only tried using luster dust once and it didn't work very well. I think I use too much vodka. How much liquid do you use with how much luster/disco dust?

Well, I am not sure there is an exact measurement for diluting luster dust with vodka. What I do is put a small amount of dust into a very small container that has a lid. I buy these containers at the craft store and use them for diluting dusts for painting because they do have a lid and it helps to keep my vodka dust from drying out when I have to step away from the project.
Anyway, then I add 1 drop of vodka at a time until I have the consistency I need. If you get it to thin your dust painting will be thin and streaky. If it is to thick it will clump and not paint smoothly. You have to find that perfect medium for your current project by adding one drop of vodka at a time. Just experiment with it a little bit and you will soon have it down pat! Once you have it down, as your mixture starts drying while you are working on your painting project, just add 1 drop of vodka to get it back to where you need it to be.
Sorry I couldn't give you a precise recipe.

I have a small painting tray that I got at Walmart. I put a few drops of vodka in one of the shallow wells. I dip my brush in the vodka, then dip it in my dust to pick up the dust, then mix it up in another of the wells, I keep dipping the brush in the vodka and working up the dust until I get the consistency I want. If I go off and leave it, it does dry up, but you can add a little more vodka and it comes right back to painting consistency.