On my 2x6 hybrid pan, I start to shut down when I have about 20 gallons left. My rig runs at about 35 gallons an hour. This usually works out well, with the head tank empty and a lower pan level than I normally run at. Next run will bring the pan back up to my operating level. This is not an exact science though, and takes some practice to zero in. You really have to know your rig and its boiling rate, cool down time, etc. I have doubled my RO capacity this year, so there may be some variation.
Gary / Zena Crossroads / 42˚ 00' 24" N / Hobby in Early '70s, Addiction since 2014
225+ taps on 3/16 (90 of which are on two Lunchbox Vac/Releasers)
12x34 timber framed sap house w/attached 10x34 shed roof for storage
2 x 6 Smoky Lake hybrid pan on oil fired Corsair arch with steam hood/concentric exhaust/auto draw-off
7.0 KW Sun Power PV System, Smokey Lake Filter Press/Steam Bottler, Modified NGMP RO - 2 4x40 posts 200 gph