To reiterate on what Justin D Wilder has said below:
When right-stick is pressed:
- Get the directional vector from the right stick.
- Multiply it by 100
- add the coordinates of the center of the screen
... and pretend the mouse was moved to that position. All your usual mouse aiming code will Just Work.
(If the player actually did move the mouse this frame, of course, the real mouse position supersedes the previous calculations.)
Not gonna lie, it looks fantastic, otherwise. I made one of these, and I'm jealous. :)