I just add a small amount of powder to the food dish and then add a little water to the consistency I need. Its not rocket science. It doesn't have to be exact. I replace it every day or 2 and most of the water has evaporated anyways. I mist my cages down every night so the water they can get directly from the glass walls of the cage. I also have a small water bowl next to the food bowl and I have seen them drink from it.
I also know people who put the powder in completely dry. And its working out just fine for their geckos.