There is a lot more than meets the eye to this, depending on plant stage, like if the plant is in Vegging state, Generative state, day or night, early after lights on or close to lights off, week of flower plant is in and Cultivar being grown. 

All that being said, your preferred medium, whatever it may be, does have a "SWEET SPOT" in moisture level to optimize plant absorption of nutrients and E.C. levels at the root zone on point. It is important to keep that level constant during those certain times of the day you are supposed to be irrigating your plants. 

The Aqua-X from TrolMaster is not only a feed vitals monitoring system but also an irrigation controller that feeds to the second to help you achieve that exact milliliter per feed if desired. There are even settings for growers using soil also.

*This Days watering chart is to display a mere reference that there is a sweet spot area of moisture, BUT In No shape or Form am I saying these are the correct moisture to air ratios inside your selected medium you should strive for.*