Edge cases when nominal distance area is zero or negative

[philderbeast]

The nominal distance area is used as a denominator in the equations for calculating distance validity. It is zero for some input values and negative for others. Examples are;

• nominal distance <= minimum distance and best distance <= nominal distance.
• nominal distance <= minimum distance and nominal goal == 0.

Please refine the set of equations for distance validity to handle all inputs. Please specify the valid domains of the inputs.