NaN = "Not a Number". It will show in one or more of the result fields when the underlying equation encounters a negative number under a square root sign, or a zero in the denominator of a division.

Leaving a field empty = entering "0" (zero) in that field.

All calculated numbers refer to a situation in which the tires can benefit from their maximum coefficient of friction, with maximum possible deceleration as a result. This situation assumes that suspension, brake system, and all other parts of the vehicle are perfectly working as intended to enable these maximum deceleration numbers.

When downforce has been taken into the equation, the calculated numbers refer to a braking action at top speed of the vehicle, since downforce increases with speed and will be highest at top speed.

These calculated numbers should only be used as indication!
Since so many (not exact*) variables are involved, the results can never be considered exact numbers.

* Coefficient of friction of the tires and brake pads, for instance. But all other variables have a certain inaccuracy too, which can cause a large inaccuracy when multiplied, divided, etc.