How to figure out router bit radius
First, setup a lamp shinning on the rounded edge.  

Get two rulers, place one perpendicular the vertical side of the stair and the other perpendicular the horizontal side.  Then line rulers up flush to each other, say at the 4” mark, you should be able to get a measurement of where the gap starts (from both directions).  That’s the radius!

The idea is you are making a small carpenters square (image below), so you see the radius of the circle.  The light shinning from behind should make it easier to see where the gap between a ruler and the stair starts.