It can be challenging to align your work accurately when reverse mounting a bowl to turn the foot. Mark the exact center of the bowl before removing the faceplate or chuck. Remove the faceplate (and bowl) from the lathe, put a dead center in the headstock morse taper, then screw the faceplate back on. The point on the dead center will leave a clean mark right in the center of the base of the bowl. You can use that mark to align with a tailstock center.