I put a sprinkle between my fingers in a small container - I use hard plastic covers that used to be on my kids' dummies, (so small), and put a little stream of water over it. It doesn't need to be an exact ratio. You can use it immediately, but the CMC probably takes 10 minutes or so to dissolve really well. A little really does go a long way. I've never done a cake whee I've needed to use more that a tiny pinch of powder.
Just remember when you are making it in BULK quantities like you are suggesting that it is wise to pour a little out into a small container to use - discard after use
Dipping your brush into your large container is just going to introduce bacteria and it will go off really quickly
Another trick for just a little glue is to put a pea sized ball of gumpaste into a new - empty nail polish bottle - fill the bottle with warm water and let set over night. In the morning you'll have just enough for your project. If you have nothing to do while watching TV you can shake the container to dissolve the gumpaste quickly.
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