FatRatKnight had a suggestion on how to figure out how long you're tapping a button:
"How consistently can you tap a button for exactly two frames? I'll give you an easy test: Training -> JV -> any track. Once the timer is running, start at 0 speed and tap A (unless you changed controls and made that B instead). 12 km/h is one frame. Jet Vermilion is done with its first gear in 8 frames, by the way, so if your speed exceeded 96 km/h, you went into the second gear's 6 km/h per frame acceleration. If you need a better feel, swap the A-B controls for a moment as a way to practice the feeling of trying the brake tapping, seeing where you accelerated to, before you go try boosting and brake tapping then (oh, don't forget to fix your controls back to normal by then). "