Thanks very much for your explanations.
Actualy I used to do the same, making copies of songs with different transpositions for different bands. But now, I've a lot of copies and it's not so simple to keep all of these coherent, but it's not a big problem, I can keep on using this method.
One thing which could be very helpfull would be to add a parameter which enable displaying tonality in parenthesis after song name in lists.
Once again thanks for your app. It's quite a miracle for us, poor musicians.