you should water entire substrate. that determines how much water you give. then wait until top inch dries in soil or if coco wait until that top layer starts to change color. repeat.
you don't pick what is needed. the wet-dry cycle that results from this reduces risk of pathogens and pests. it also promotes deeper roots vs superficial roots. if oyu try to add a little water on top you will have a bunch of superficial roots that are not as effective, and if exposed to light won't even be roots for long.
always water entirety of substrate.. a little runoff is fine. cast it down the drain. don't let it sit in its own piss. in a soilless context, you want 10% runoff every time and fertilizing every time. soil s a bit different bit in the end similar. you'll learn how much is needed and limit runoff in soil.
if you irrigate at same loss of weight / dryness up top, it will require the same volume of water as before. you learn how much retroactively. use the same soil / pot size / vege time next time and it'll stay the same - even ramp up in frequency in same way relative to growth / pot size.
plantslave 
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