The origin is the upper left corner. So to get the center you have to take the views width and height into account. The center of the screen is at position (Width/2.f, Height/2.f). So if you want to have that one as your origin you have to transform if from that position subtracting the origin. For example:
You have a screen size of 8x8 (width x height). The center is so the point C(4,4) while the origin is still O(0,0) which is the upper left corner. Now you get a mouse location of A(1,2) relative to the upper left corner C(0,0). To transform it to be relative to the center just subtract the center from that point (the new origin): A’ = A - C = (-3, -2) which is exactly the relative from the point C if we take it as the new origin.
You could also flip the vertical (Y axis) in case you have to. You can get the Viewport size from any PlayerControler with a function called GetViewportSize (or similar :D).