Style of riding -- whether your stance is more forward or more duck; relative angles -- whether you're more pigeon-toed, duck-footed, or strictly parallel; canting -- if your bindings support it; stance width; and set-back. There are a lot of variables. Fortunately it isn't like setting up a nuclear reactor -- there isn't one correct setting. You mostly set them up by guess and try it out. If it works, it's good. If it feels uncomfortable or feels like it's interfering with your control, change it.
I got my binding angles early on -- I've been at 15/-9 forever. But I have fooled a bit with set-back and stance width. Works fine now.