Converting measured voltages, in dB or in volts, to the subjective assessment of loudness is - shall we be nice and say, "imprecise"? :^)
The best way is to use the existing controls since they have known step values.
The lowest setting is "off"
The first step should be very quiet - useful for when the phone rings.
The second step is 12dB louder - intended to be for late-night listening.
The third step is 6dB louder than that.
All the rest of the steps are 3dB each.
An easier approach is to start with your observation that the lowest level (first step above zero) is as loud as you want. That step is 39dB below the highest step. I'd start with the maximum supplied attenuation, 18dB using the 360K resistor. That my or may not be enough attenuation - you must have a high-output CD and phono setup, or sensitive speakers, or both! If that's still too loud, you could use a larger resistor, up to 1Megohm, but you'll probably do better to reduce the Paramour gain - see my white paper linked off the Community page for more details.