My criteria for water misting is pretty simple.
If you run your hands through dry malt, you will be left with dust on your hands. I sequentially mist water onto the grain and mix by hand. I keep doing that and checking the dustiness of my hands each time. When my hands no longer have dust on them, I consider that: good enough.
Remember, the worst thing you can do is to oversaturate your grain and mill it. That is when you will gum up your rollers with wet flour. No fun to remove.