I have very nearly mastered the clarification process but still wish to control the timing of flocculation more consistently. At what point/stage is cold crashing most efficient?
I am not sure exactly what you are defining as efficient, so I am going to answer this assuming time efficiency is your primary goal.
Denny's advice is good advice for a general approach without having to faff around checking things, and will ferment all but the largest brews to FG.
I would suggest you take daily samples and gravity readings. As soon as you have 2 of the same readings, then I would start the crashing process for optimal time efficiency. You can cold crash down to -1°C/30°F, as the alcohol and sugars in solution will prevent freezing, but you should be aiming for 33-38°F/0.5-3.5°C.