Since Franck's organ at Saint-Clotilde only had one expressive division, the Recit, I didn't use any Positif expression at all and left the shades fully open throughout the piece.
A word of caution about the Recit expression in the recording versus the MIDI file...
In voicing my PAB instrument, I reduced the amount of volume change from open to closed expression shades. In playing the MIDI file on your instrument, if you have left the Recit expression at the standard settings, the diminuendo will be greater than you hear on the recording and how it sounds on my instrument.
I just always felt the dynamic contrast between open and fully closed expression were too great in both the Positif and Recit divisions, so I reduced the amount of change in both divisions. To me, it sounds more natural with the dynamic contrast not as great.