You can use the Flip tab of the Transform dialog box to flip an object at a specific angular value.
When you flip an object numerically, you specify an imaginary line that passes through or beside an object at any angle. The object behaves as though it is hinged to the line, and flips over it.
To flip an object at a specified angle, select the point as which the flip occurs from the Origin list box, and then type an angle for the flip axis in the angle box.
You can place the imaginary axis at which flipping occurs at any of the ten object origin points. For example, select the center origin to flip an object around a central axis, or select the right middle origin to flip the object around its right side.