Generally it is possible to generate a “custom†linear equation in the form of y = mx + b for any linear responding instrument relating input directly to output, a more general approach may be used to relate input to output values by translating all values into (and out of) per unit quantities. A “per unit†quantity is simply a ratio between a given quantity and its maximum value. A half-full glass of water could thus be described as having a fullness of 0.5 per unit. The concept of percent is very similar, the only difference between per unit and percent being the base value of comparison: half-full glass of water has a fullness of 0.5 per unit