Measuring the EC of what drains out of a coco pot tells you whether salts are building up before the leaves complain, and with an auto that early warning is worth having. If runoff reads higher than what went in, the next watering should be plain or diluted until the numbers come back together. Target inflow and runoff ranges by week for autos are listed on https://turboautoflowerseeds.com, and they run lower than the photoperiod charts most pens ship with. A cheap EC pen pays for itself in one avoided lockout.