Are you asking how to dimension to quadrant points of arcs? Hold down the shift key while you are placing the dimension. SW will snap to the quadrant of the selected arc.
How to create a centerline? In your case the actual centers of the upper and lower arcs are not concentric. SW will place them in the true centers so you will see an upper one and a lower one.
If you want to place a centermark at the implied or symmetrical center you will have to manually draw centerlines at that point.
I suspect that this part is a shaft coupling. If it is, and it is made by cutting one part in half, then the common practice would be to show it as one complete part and indicate the width of the saw cut in the middle. That way both halves share a common center.
