Pick the medium and the stage: field capacity by substrate — coco 75%, rockwool 80%, soil 45%, perlite 35%, clay 50% — decides the water bank the dryback rolls against.

Set the dryback target: the percent of that bank you want transpired between events, 10–30% for the working band, higher with higher EC for generative steering.

Compute irrigation time: substrate litres × dryback ÷ (flow L/h ÷ 60 × emitters), scaled by stage (seedling 0.5, veg 0.8, flower 1.0, fruit 1.2), VPD factor and root heat (+2% per °C over 20).

Adjust events per day: base frequency × VPD × (1 + 0.05(T − 20)) — a dry hot room waters more often without a manual change.

Close the loop on runoff and moisture: runoff = (shot − substrate × (1 − dryback) × FC) ÷ shot; moisture = 100 − dryback actual — no drainage means every fed ion stays in the pot.

Read the safety band: dryback 10–30 %, runoff 15–25 %, moisture 50–75 % green; the red edges are dryback <5 or >45, runoff <10 or >35, moisture <40 or >85 — and the EC drift line tells you what no-runoff is really costing.