My maths is a bit crap, but x seems to be the unknown factor. When I write first drafts, XXX stands in for names and places I haven't yet decided on, or can't remember what I used earlier in the book. But I notice that the same first drafts have fairly simple characters at first, that slowly gather history and mystery as I write on. The main characters often end up quite different at the end of the draft than they started out. I find the mystery, and the second draft becomes a whole rewrite. Now I know them better they will behave differently, and possibly take the story in a completely new direction.
I like characters to be complex and involved, like real people. I used to meet people through my work at a hospice. These weren't people who would normally have counselling, but their bereavement was a stress so I would see them. Under the most conventional exterior you could find the broadest range of personal secrets, tastes, lifestyles and emotions. It all entered into my memory banks and sometimes (completely anonymised, of course) into notebooks. Out of that mass of observation come characters, sometimes so real to me they almost dictate the story.
I wonder how much of those 'real people' creep into the books, and whether they (or I) will recognise them.