I had the 5-shirt Jaquard tie dye kit, and I was able to dye 5 flour-sack flats and 1 prefold. You can figure that a prefold will take about as much dye as a t-shirt, depending on how wet or dry it is when you apply the dye. (The wetter the item after soaking in soda-ash, the less dye-saturation. This means more white spots. If you allow the tied item to dry a bit after soaking it in the soda ash bath and squeezing it out good, you'll get better color saturation, but it sucks up a lot more dye that way.
Personally, I think you'd be better off playing with your tie-dye kit in the way it was intended, have fun with it, and then buy procion dyes in larger quantities from
www.dharmatrading.com for your vat-dyeing venture.
I use just the 3 primaries for my mixing, mostly. When I'm mixing the colors, I do it after I've mixed up the dye solutions, not while they're still in powder form. I'd say about 1/10 turquoise & 9/10 yellow gets you a nice spring green - more turquoise will deepen the green toward kelly. The same proportions of magenta & yellow will get you a good orange - it's easy to go too heavy on the magenta, and you wind up with an orange that looks too red to be recognized as orange! Purple is trickier - if you don't add enough turquoise, it looks too wine-ey. I'm still not that hot with the proportions for purple! I'd start with half a bottle of turquoise, and keep adding magenta 'till you get the right shade. If you look at how full it is at that point, that will give you a good general idea of the proportions to use.
