Since it opinion I will throw in my two cents.......I grow mostly in 5 or 7 gallon containers depending how big I will let a cycle get and it is at my age the biggest I can pick up and move around. My soil mixture hasn't changed much in years so I only have to do this when I change my mixture to know how much water my soil will hold.
I like to water from the bottom mostly and allow the water to be sucked up by the soil. In my situation my mixture holds about 5 quarts of water when allowed to be soaked up. I place the container in my tub and add water to the soil and then allow the soil to soak up the run off. If it doesn't soak that up I know I am watering too close together. Time consuming and not for everyone but sure works good for me.