I managed to do it by simply sending a number in the dac~ object.
If it can help anyone :
Notein
expr (($f1-36)*0.01675)-0.201)
dac~
In the expression :
$f1 is the midi note number
0.01675 is the value for a semitone step. This is an approximation due to the fact that when I measured the voltage a the output of my soundcard, it wasn't consistent octave to octave, so I settled for this for now. I have yet to measure the deviation with a tuner.
-201 is a kind of octave switch. 0.201 is the value for an octave that I settled for that was giving me a steady division for a semitone step. If I substract it at the end of the expression, it allows for a lower octave range by offsetting the value. Example : do not put it in the expression, you get the straight value. -0.201 gives you an octave lower than the setting on your synth (16' becomes 32'). -0.402 outputs two octaves lower than the setting on your synth. Adding 0.201 gets you in the other way.