If one attempts to figure out what the subjective, perceived numerical volume vs. actual dB SPL is for a large group of people, it roughly coincides with the dB SPL number.

Bit more detail: you play a 100dB SPL tone and tell the participant "this is a 100" and play a 0dB SPL tone and tell the participant "this is a 0" and play all kinds of other tones at various frequencies and sound pressures, you get that curve. The unit for this arbitrary scale is called phon.