Math.atan2 to make sprite follow mouse

im trying to make an object always face my mouse, trouble is getting a radius from atan2 based on the mouse position take it from the top corner of the screen, how can i move the point of origin to work out the mouse position relative to the object i need facing it?

The point of origin cannot be moved, but you can use the top corner of the screen as a common base of reference for both the object and your mouse.