Tablet joystick appearing on touch

I am trying to do the same thing as this guy does, but I'm having a different problem: I have modified the joysticks a bit to create a joystick base and not only the stick itself. While I've managed to make the base area appear wherever the player puts his finger, the stick appears in the upper right corner because for some reason its Zero is in the bottom left corner of the screen instead of the center of the joystick base.

Any ideas?