For vegetative growth I would use a 18/6 18 hours of light and 6 hours of darkness lighting schedule. I read a study where lighting schedule versus root length was tested and plants grown under a 16/8 lighting schedule grew the longest roots, 18/6 was not tested. I was made aware of a study where lighting schedule versus growth was tested and plants given at least 5.5 hours of darkness grew the most.
I'm not arguing that study in the slightest bit possible. But if a plant is constantly photosynthesizing the roots are forced to grow, thus the more needed nutrients to support that growth. I've done my own experiments on this with little clear solo cups. I did this with seeds. The seedling under 24-0 had its roots wrapped around that cup 1 week earlier than the one under 18-6.
Maybe it was genetics that made that variable in the study you read. Maybe genetics had a part in my study. But as it stands more photosynthesis = better root systems. In my mind at least.