ElectrifiedTracker

[Contagious]

You have to remember the global cooldown. If you cast a different spell while the timer is in the white area (which is 1.5 seconds long) then the gcd will start. The gcd lasts 1.5s so as a result your fl/tb will start casting in the red area, too late.
It is true that the perfect timing is the edge between white and red, but only if your last spell was cast on the edge between white and black.