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If you're producing units constantly from 3 warpgates, you need to add pylon after a cycle.

3 warpgates = 6 supply per cycle
Since build time for 2 probes = 1 warpgate unit build time, thats 2 supply per cycle.

So, when you select "W" then press S Z E -> select 4(my nexus hot key) the press  "E" "E" -> put a pylon. Then repeat.

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If your producing from 6 warpgates and 2 robo, let say you're making colly for PvZ:

(2 colly x 6) + (6 wapgates x 2) + 2 probes = 26 suply or 3/4 pylons per cycle.

6(my hotkeys for robo) "C" "C" -> "W" S S S S S S -> 4 "E" "E" -> straight put 3 or 4 pylons. This is the rough estimation of the build time , so always look at your supply count.