MIDI notes range from 0-127
C0 = 0
C1 = 12
C2 = 24
etc…
v/oct is linear, so
C0 = 0v
C1 = 1v
C2 = 2v
etc…
If you are setting C2 to 3V, you are shifting everything up one octave.
I personally have these settings and track external gear just fine:
Note 1: C0
Voltage 1: 0v
Note 2: C3
Voltage 2: 3v
You will likely have to tweak from there depending on how your external synth tracks. I haven’t tested the full range of accuracy, but that gets the job done on for several v/oct synths for me.