Here is a old post but he did some testing. He say 20/4 has higher yield then 18/6..
https://www.rollitup.org/general-marijuana-growing/244146-difference-between-24-7-18-a.html#post3107354
https://www.rollitup.org/general-marijuana-growing/244146-difference-between-24-7-18-a.html#post3107354