Ok so the plant has evolved to put the most energy into the top that is highest. This is because its the most likley to be polonated and reproduce. Your goal should be to train your canapy to be even height. So that energy is put into all of them instead of just one or 2. Any top that isnt will be a much smaller flower. Many L.S.T to achieve that. Some prefer H.S.T or supercroping. The key is to do it early and often. Keep them tops the same height.
Remember your garden will double at least in the first 3 weeks after flip. Only you can say if its time or not. As for ph. Idealy it starts at 6.2 6.3 and drops as the plant matures. Should be 5.9 6.0 by transition and finish at 5.5 to 5.7
Ppm depends on much more. Type of hydro system, how much light, co2, ect