I hear a lot of articles saying those 3 nute numers ( like you said 20-15-20) are important in growing... The higher the first number in the beginning of the grow will help better then once you are into in like 8 or so week go to a higher 2nd number... as for the 3rd number i don't know... higher 3rd near harvesting??? lol But I've heard about this and some of my friends do this and they actually are seeing better results and a better smoke if you follow that...
Remember higher 1st number in growing/sprouting
higher 2nd after 8 to 12 weeks into it
and My opinion 3rd higher near harvest.. no one ever says anything about the 3rd number so thats just my guess ha.