This is how I do it , I have my own organic soil , so what I do is water to a little bit of run off and see how long for the plant to uptake the runoff , then add a litle bit more , wait 10 min and see what is left , taking note on the amount of water used , you may have to add more or less , but remember the plant will use more as time goes by . It is a little bit of trial and error , but once you get the hang of it you will not have to worry about it .