Finding out how much water a plant needs.

Your approaching the idea wrong. Nutrient can only uptake where it finds water. Problem is not how much water but how long it will sit within the medium before it stagnates. Moisture within the medium only escapes if it is uptake by roots or evaporated. Rh transfers from one atmosphere to another , if your medium is moist and the surrounding air is dry, the air will draw moisture from within the medium slowly over time. Creating a negative pressure within the tent allows one with enough experience to dicate that no water will remain within the medium after x amount of time. If you use airpots it's practically impossible to overwater. I like to grow organically so there is 0 chance of salt build-up or burning from nutrient. Most overthink the watering, truth is the plant will unmistakably droop all it's leaves at least 24 hours before any permanent damage is done. It keeps about 10% of its water reserves before it droops, giving you that window. So as long as you check in at least once every 24 hours you can never actually damage a plant or really slow its progress from.