Someone please double check my memory and calculations . . .
The length of the circumference of a circle is 2 times 3.14159 times the radius, or 3.14159 times the diameter, here 20", so the circumference would be 62.83 inches. If the circumference is divided into 5 equal segments each would be 12.57" along the curve.
There are 360 degrees in a circle, so I would think five equal pie shaped pieces would each have an interior angle of 360 divided by 5, which my calculator says is 72 degrees.
A triangle with one vertex at the center of the circle and the other two at opposite ends of one of the segments on the circumference would be an isosolese (how is this dad burned word spelled anyway) triangle, meaning the outer two angles are the same. There are 180 degrees in a triangle, and if the center angle is 72 degrees then each of the outer angles would be one half of 180 minus 72, or 54 degrees. The angle setting on a miter saw may be the complement of this angle, 90-54, or 36 degrees.
Finding the straight line length between the two ends of each segment would require a trig calculator or table, and I have neither here.