Wait until there are small "budlets" before switching to bloom nutrients.
I usually stay with grow nutrients for the first 2 weeks of flower, before changing to half bloom and half grow for the next 2 weeks, before changing finally to straight bloom nutrients through until harvest.
This seems to produce the biggest blooms on plants that don't exhaust themselves and end up a sickly yellow by harvest time.
Have been doing this for 25+ years and swear by its effectiveness.
I also only use half to two thirds strength nutrients too.
Hope this helps, Organoman.