I hate to admit it but I shift totally by ear. Once I have driven a car
a lap or two, watching the tach or the idiot lights, I know what pitch
represents redline and what pitch represents the bottom of the power
band. So I do both upshifting and downshifting by ear. Generally you
want to stay in the power band (keep the lights lit up in modern F1
cars). The exception is in turns, where in slow and medium speed turns
you don't want to be in the power band until you are in a position where
you can begin straightening the wheel. So for those types of turns skip
the last downshift and let the engine drop below the powerband in
midturn. If you have your gears adjusted correctly you should be just
coming back into the power band somewhere near the apex.