I'm not at all well versed in music theory, Simon1WP; but I am experienced in the ability to hear what notes I want to use when I'm writing a song.
How do I hear something nonexistent without using music theory?
I was born with the natural gift to hear music and sing songs. I cannot take credit for it.
You may have the same gift, for all I know.
Sometimes all you need do, is be in a quiet place and think how you feel about whatever you are writing your song.
Do you think in terms of sadness? Or happiness? Or anger? Or love? Or Peace? Or War?
Those topics evoke a certain "feeling" for me, and I've written poetry first to express those feelings in words.
Then if I "hear" a melody in my head, regardless of the "key", I apply that poetry to the melody. Once I have sung about 4 bars (measures) counted out with the first line of the poem, I know that I have managed to find the notes, to what I'm writing.
The next step is easy. I just sing the melody in the key I wish it to be in.
In your case, lets say the key of A major.
From there, the melody takes me through the progression of chords that I "feel" the song should be in.
I know this process may not be your choice, and you may prefer to find a melody by randomly
trying notes in the numerous scales that can be derived from the A Major key, etc.
But I'm just saying that regardless of what you do or how you it, Music WILL bring you a "feeling" that is not necessarily a mathematical "thing".
The dissection of a song; even while writing it, can try your patience when you struggle to "find"
the notes you want via music theory.
I am not disrespecting music theory here...at least not on purpose.
I'm just saying that before music theory even existed, people were singing and making sounds
that could only originate within the heart & soul....the mind came later.
Please don't take offense at my comments here.
I'm just expressing my feelings of how I understand music.
Let us all know, through this site, what you come up with. I'd like to hear it.
Former lead singer for The Human Beinz