My experience has been the exact opposite, at least with ProMix HP.
I filled my reservoir 24 hours before transplant @ day 20 (?) and noticed the bottom of the prepared hole was barely moist.
About two weeks later my plants were starting to lean. The top 3 - 4" of the medium were dry.
I figured there was a healthy tap root but no side roots to help support the plant so I started to water from the top every third feeding.
5 gallon pail with GroBucket, when the dip stick showed one I fed a gallon from the top which would produce about a litre of run off from the drain hole.
If you drill your drain hole in the correct position there will always be about 1 inch of air at the top of the insert.
This seems to be the trick !
So, totally saturated ProMix HP and Full reservoir every third feeding or so and my girls were Praying !
Six Shooter Auto, 48" high, bud back building (cut once) wet trimmed @ day 132 !
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In my drying "tent"
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Cheers