...The rest of the time it's all by the sound of the revs as I try to keep the engine in the power-band.
I think most folks who ride a bike or drive a standard-shift car do it by 'feel and sound'... I know that's true for me, and if I 'quote' where I shift, that's really just working backward from years of experience. I shift when it sounds like it's time to shift - depending on how I'm driving right then. If I've come to know that means X,000 RPM when being mellow and X,000 RPM when being aggressive, that's just working backward from the 'feel' of it.