Remove the base plate. Clamp two pieces of wood parallel, exactly the diameter of the router's base. Check by fitting the router between at both ends. Once set parallel, remove the router.
Divide the distance between the two pieces and make marks wider apart than the diameter of the router base. Draw a line connecting the marks. Double check to make certain the marks are precisely centered. Place the router over the line, between the boards. Place the router between the boards and use a marker to transfer the drawn line onto the router's metal base. See the diagram below.
Replace the sub base on the router, using a centering pin (Pix below) to center the sub base. You could mark the sub base with a white fine point marker, but if you remove the sub base you will want to re-center it.
The more I do, the less I accomplish.