[warrenn]

Why not use the arc tool? Let's say you want a horizontal semicircle. Place a horizontal arc (drag it from shapes) on the canvas and click on the end points, extending each end until the angle is 90 degrees (indicated in the little box). For a quarter circle, set each end at 45 degrees. To change orientation, hold down command key and click on resize box - it now allows rotation by any angle, indicated in a little box. The problem with the corner radius approach is that the shape has a control point and moving this can distort the object - also, you don't know when it is exactly a semi or quarter circle. Finally, with the arc approach, you have the center, which is a cardinal point and can be used with alignment guides (I think).

wn