18/6 is the maximum efficiency for cannabis, any more is just a waste of electricity.
Welcome to the world of auto flower disappointment, they are not all they are cracked up to be. Spent 2 years wasting my time with them and have gone back to photo period plants for ever, never will an auto get planted by me again. Stick with the photo period plants - much better yield, much more stable genetics, less electricity to grow, less whimsical, generally more potent and causing much, much less problems. Nearly every question in this forum is about problems with auto flowering plants, which tells me a whole lot.
Hope this helps,.... Organoman. (sworn enemy of autos!)