you can always add more water, but cant take out water once its in. soil at least. I always underwater, if the plants seem sad, i give a little more. and surprisingly, when I underwater, my plants droop like that, then I give them some more and in 4 hours they perk up. but yeah, at this point i dont think about it, I go completely by the weight of the pot, because each plant is different, i had a plant that drank once a week, and another that needed once a day, i just sorta tilt the pot with some fingers and if its light water.