Just caught up here friend, nice setup I'm going coco next too.
From what I read your supposed to feed every time to 20% run off
And never let them dry out because the salt buildup is real in coco but you really can't over water once their established coco just doesn't retain enough water to drown our plants.
When you flush with straight water, the plant and medium seek an equalibrium so by having low ppm nutes you actually draw nutes out of your plant.
I'm sorry also I missed if your doing drain to waste or recirculating.
I hope it's drain to waste?
Are you using tap water or reverse osmosis?