Delcour,

Thank you for your contributions to the Liturgy. If I may be so bold – I believe you will eventually find it possible to accompany the choir directly from the Graduale – from just the melody. Since the four-line chant notation does not indicate specific pitches, but only intervals in relation to Do/Ut or Fa, I personally find it easier to transpose the chant accompaniment to another key when playing from the Graduale, rather than from a written-out accompaniment (e.g. Peeters). Then I only have to read one line, rather than the four parts in an organ accompaniment.

Perhaps you might find this a more effecient solution than the long and difficult work of inputting chants into Finale. You could practice this by playing over Peeters’ harmonization of a given chant, then playing that harmonization while looking only at the melody, noticing especially how the cadences are made in that mode. Then try it in another key, still watching the melody and thinking about the cadences. Then go on to another chant in the same way, perhaps in another mode, and get used to how that feels, and thus become accustomed to that style of harmony, and soon you’ll be good to go.