Tread very carefully Nard, use of unauthorised software for flight operations is walking a legal tight-rope!
I have written a lot of software for aircraft performance, and in each and every case it had to be approved by the relevant regulatory authority before it may be used. This will basically get right down to providing the authority with the programme, which always worried me because the ONLY time that I was ever plagiarised was by a regulatory authority (true story!)
On the other hand, if you write a programme for YOURSELF, and do not distribute it, then that's OK, just as another pilot is allowed to compile and keep a set of written notes for his or her own use.
Be very very careful..........................