Q: “Where should my right foot be relative to the throttle and brake?”
A: Heel on the floor, in a position as close to being right in between the gas and brake pedals. This way, you can pivot or lean your foot to the right to use the throttle, and pivot/lean to the left for the brake. When braking, the ball of your right foot should be applying the brake pedal – which can feel odd if you’ve always applied the brake with the pedal in the middle of your foot.
It will take time to get used to this feeling that your foot is not going to slip off the brake pedal, but you’ll find that your foot is only on part of it – and that’s okay. In this way, your heel barely moves, and your foot just pivots/leans from right to left and back again as you use the pedals.
Some drivers have smaller feet, and might be driving a car with pedals that are high enough above the floor that it makes it difficult keep the heel on the floor. If that’s the case for you, and you’re driving a designated track car that you’re okay with modifying, I’d recommend building up the floor enough so the ball of your foot is on the middle of the brake pedal pad while your heel is still on the floor. This may involve mounting a plate on small “stilts” or spacers to raise up enough the area where your heel rests.
As I mentioned, it can take a bit of time to get used to this. In fact, that is usually a bigger factor than having a smaller foot, and with some practice you’ll feel comfortable applying the brake with the ball and toes of your foot. Doing so will result in much more sensitivity to applying and modulating the brakes, as well as a quicker and smoother transition from throttle to brakes and back to throttle.