I just eyeball it and make sure the water is incorporated into a well mixed slurry, you have to watch it because it expands quite a bit while microwaving and can overflow a bowl if your bowl is too small.

If you have too little water before microwaving it, it won't expand but you'll need to mix in more hot water afterwards otherwise the texture will be subpar.

Barley is cheap, I'd just start with less than a quarter cup ground powder and go from there.