Their flowering mechanism is not based on light at all but an internal timer set by genetics which you can do nothing to delay, they do what they want. Any light leaks won't change your flowering cycle. Technically you could flower it under 24/0 lighting, but a waste of energy imo.Since autos are not photo dependant I assume that some ambient light entering the grow space during lights off is okay?
Or do they need absolute dark also?
20/4 is perfect! This is the light timing for my autos and they are doing good. Some autos may take up to 12 weeks so dont worry if they pass 10 weeks. Be patient and enjoy the grow18/6 is good some go 24/7 and suffer ill effects later,
short growth and poor development
to my blog: on hints https://www.rollitup.org/Journal/Entry/10-top-tips-for-a-better-autoflowering-yield.30162
good luck