Percent of volume says nothing about the line level, which is hardware specific. Since audio jacks were invented before we had industry standards bodies, there is no specification that hardware manufacturers have to meet. Many provide around 1Vpp, or -10dbV, at max volume, but it's going to vary from device to device. The impedance of the headphones are going to affect the SPL as well, and none of it is likely calibrated to any high degree of certainty unless you're buying professional equipment.
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