I am just using the 4100K bulb from start to finish.
Not sure about that 20" thing though.... This bulb at 20" stunted my plants severely, they were just over a foot and just straight up.
Very odd and not used to that.
Next seedlings i raised the light to about 3 feet and they started to stretch and grow and eventually i ended up with the light as high as i could get it, about 6 foot.
Using one of those cheap light meters, it still maxes it out at plant top level and the plants are so much happier!
Also learning that growing just one strain at a time may be the way to go... Thinking of ease of feeding and care instead of mixing per plant requirements.
So after a very meager first attempt, the second crop is growing much more to my liking!
Two feet tall and round and bushy with lots of bud sites forming all over.
Light is about 3,5 feet above plant top, possibly more.