While treating Spock for a neural parasite infiltration in 2267, DoctorMcCoy activated this indicator to illustrate to CaptainKirk how much pain the creature was inflicting. The indicator rose variably between 75% to 85% of its maximum; McCoy commented that it was "no wonder the poor devils go mad", implying that the pain indicated was severe. When Spock continued to speak, the indicator increased to nearly 100%. (TOS: "Operation -- Annihilate!")
The Star Fleet Medical Reference Manual describes the K3 indicator as displaying the "electrical activity" of the brain of a patient, while conceding that extreme activity may indicate high levels of pain. This means that it would not be considered a true "dolorimeter," or pain-measurement device.