I use an old ice block tray for my Spotone. I let it dry as described above usually in different dilutions which I mix just using the brush itself. You only need tiny amounts. When I am going to do some prints, I put one drop of water in another dimple and use that with the brush to dampen the dried Spotone as needed. Once I'm finished, I dry the water dimple and put the tray back into a plastic bag. I used to put it in a box but it got covered in dust too easily.
One thing I find is if you try to match the tones exactly, you're more likely to make a mess of it (I know people say you can wash it off but I've never had any success doing that... it doesn't seem to come off!. Usually, I just aim to knock the brightness of a spot down enough that I can't see it from a reasonable viewing distance. I creep up to the tone very slowly, letting it dry before deciding if it's right. I then get the quality control officer (my wife) to inspect.. if it passes her anaylsis it's pretty good
However you do it.. unfortunately practice is the key. The cure of course is not to get them in the 1st place!!!!