Actually, you can do it if you just count.

Click on the rotation handle and hold the SHIFT key down. As you drag the rotation handle, the shape rotation will snap to 15 degree increments. When it hits the third snap you are now at 45 degrees. If you keep going, it is another 3 snaps to 90 degrees.

Another hint for better control of the snap positions is to move your mouse further away from the shape as you start dragging, then the 'leverage arm' of the rotation is longer and it is easier to see the individual snaps because the mouse movement is slower to rotate the shape. If your mouse is closer to the shape, small movements of the mouse could cause a double or triple snap which you might miss-count as a single snap.

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Andrew Lockton
Chrysalis Design, Melbourne Australia