Force on brake pedal for maximum deceleration has to feel comfortable for you, the driver, when repeated several hundreds of times during a race. For the average driver a common figure is 75 lbs (34 kg). Some drivers like a brake pedal that is "firm", which usually means they are satisfied with a maximum pedal effort of around 100 lbs (45 kg).

Bottom line: Choose the number that you are comfortable with; the rest of your brake system can be designed to work with that number. Keep in mind though, that if the car is being shared with other drivers, pedal effort should fit the lightest or least strong driver.

Note: The above information applies to race cars. On the other hand, road cars are designed to be comfortably and safely used by any adult. For example: a 95 lbs (43 kg) person should be able to "brake into the ABS" just as easy as any other heavier or stronger driver. Therefore, on standard road cars, you might find that for example 45 lbs (20 kg) is the number that is required to achieve maximum deceleration for that particular vehicle.

Also: Energy from the rotating mass of a car represents roughly 10% of the total kinetic energy that needs to be addressed by the brakes when they are called upon for help with slowing down a car.

This calculator doesn't take rotating mass into account because it's a factor that can vary widely from car to car and between vehicle styles.

Knowing this, we can at least aknowledge that the force on the brake pedal necessary for threshold braking will be a little higher then the number that's shown in the field for that parameter. Probably somewhere in the neighborhood of 10% higher.