Playing devil's advocate for the sake of discussion:
The units in table A-13 "Sensor Unit Defines" are all derived from the SI Base Units (Meters/Kilograms/Seconds). In this system, a percentage is a unit-less value, thus UNITS_NONE is the most technically correct.
A percentage is unit-less because it is the ratio of two values with the same units. Thus the two units cancel out resulting in a dimensionless value.
For a fog fluid sensor, you could define a SENS_VOLUME with UNITS_NONE and a description string of "Fluid Level (percent)"
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