I Would Not add bushmaster with nutrients. Clean slate, RO + Bushmaster for 24 hours. In my opinion the plants have enough nutes stuck to their roots to keep them going for 24 hours in just RO. The Bushmaster and it's sister products dramatically increase nutrient uptake so the 24 hours is no problem.
Even when I use gravity, I think I keep the ppm around 300-400 in the 5th week of flowering. That's down from 750-850 before and after the gravity use.
This method has worked like a charm, and I have never nute burned my plants. Also, I've not had to raise my light at all using gravity... I don't know what that is attributed to.
As for flowering... I personally consider the flowering time to begin when you see preflowers, or whiskers. I have been bashed by many for thinking this way... the popular answer is when you switch to 12/12.
I beleive it starts at preflowers for two reasons. One internal and one external. I looked up on two different seed company websites that when they say 8-10 weeks finishing time for a strain, they are starting counting when preflowers show. However, I found another seed company site that said the opposite. The first two seemed to be the most reputable. I don't remember what the sites were... if you really want me to find them again I can.
The internal reason I beleive the preflower idea is this. I feel like the mj plant is not a machine. It seems improbable there is an internal switch in a plant that at one instant switches. In a natural setting a plant would slowly transition through grow cycles with the seasons. In our closet grows we do have a switch (or timer) that changes season in an instant (the instant we reset the timer). I don't beleive the plant knows we do this. I think the first time there is 12 hours of darkness the plant says to itself, "huh... guess it's a long night"... the next 12 hours of darkness it might say "well, I guess it's fall already, that's strange". Then after a few days to a couple weeks the plant has settled in to the fact that all of a sudden it's fall. This is when it starts working on flowering... and this is when I start counting my plant as flowering.
This is also the point I start giving the plant strictly bloom nutrients. As the time between 12/12 and preflowers the plant is still stretching some, still wanting some veg nutes. So the time between is what I call a transition.
I give the plants 36 hours of darkness going into 12/12 to jolt their system. Make them think that 36 hours is when late summer and early fall came and went. Then when the next darkness lasts 12 hours they say "huh, guess that long period of darkness meant it's fall now". I hope they are getting the message faster.
I know I can make things to complicated sometimes, but I enjoy it... and isn't that the point in life. To enjoy it.
Thanks for asking about the BM. And about the flower thing.