Wow,
I used GH flora 3 part my first grow and went by instructions. was using tap and I over nuted them in veg. Switched to R/O and still too many nutrients. dropped to half and they stayed there till finish and were great.
I have switched to the nova series with additives and I find about half of what is recommended works for my strain. I think it depends on what you are growing. Find out if the strain is nute shy or not. I was growing Apollo at the time.
Widow, Dragon, and Haze is not as shy. I would start at half and work up from there if I were you. Never hear of people under nuting plants do ya?
After a few grows I found that flora series is hard to flush in the end so I veg with flora nova series and flower with Fox Farm. They are organic and flush out fine in a week.
Oh, be sure to ph after you add nutrients
Good Luck,
GH practaly wrote the book so good place to start.