The volume levels on the original console increment in a logarithmic scale - approximately 2dB per unit set in the volume register which has possible values ranging from 0 to 15.

dB = 20*log(V1/V2)

and thus:

(V1/V2) = 10 ^ (dB/20)

10^ (-0.1) = 0.794328 ratio between any adjacent linear volume levels. This can be used to build a volume table which I will verify with oscilloscope measurements.

Some emulators use the following volume table:
int volume_table[16]={ 32767, 26028, 20675, 16422, 13045, 10362, 8231, 6568, 5193, 4125, 3277, 2603, 2067, 1642, 1304, 0 };

which corresponds to volume register values 0 to 15, in that order.