My recent escapades in watering (soil, not coco, or other mediums) have led me with full sized plants to not worry about putting in too much at one time, but to be more concerned about letting 'em dry out before you re-water.
I dropped 5 gallons of water through one plant last Friday...I get runoff starting at about a gallon. So what that means is there was ~4 gallons that went in top and back out the bottom. The plants don't care how much is passing through the pot in all honesty. I have houseplants that I water by putting under the faucet and just letting the soil saturate and keep running until it's pouring down the drain so I'm not new to this concept.
What you end up with is a substrate that's fully watered (minus errant pockets that this helps to get rid of). Then I don't do anything until I dig a finger in and feel dry down (for me that's 2-3 inches). My soil is too dense and should have more perlite for quicker drying...but Friday's 5 gallon water (really a flush), meant that I re-watered that plant yesterday (Wednesday).
Unless you're with young sprouts and seedlings, overwatering isn't 'I used too much at a time' as it is "I've watered too many times."
Also, if you're not getting the soil wet everywhere, roots aren't going to populate those areas. They want to go where there's moisture.
