The principle that the just noticeable difference of any given sense is a constant fraction or proportion of the stimulus being judged is called

This principle is called Weber's Law of Just Noticeable Difference.

Explanation:

Weber's Law states that a a just or barely noticeable difference in a stimulus is constantly proportional to the original stimulus. It applied to various sensory stimulus such as loudness and brightness, amongst others.

For example, the brightness of a phone during daylight is barely noticeable since the light is proportional to the brightness, however at night the brightness is way more noticeable since the darkness is not proportional to the daylight.