There are a bunch of router round over bits laying on the bench.

Quick, which is the 5/16" size? You grab the ruler and try to measure the curve. Then you try measuring from point to point and apply the formula. Something or other about a square root. (All the roots that I've ever seen are round.....)

So here is the secret. Measure the total diameter of the round over bit. Subtract the diameter of the bearing from the bit diameter and now divide this number by 2. The result is the size of the round over.

The measurements are best done with calipers. Harbor Freight has a plastic (puke green and maybe fiber glass) vernier caliper in the less than $3 range. These calipers are reasonably accurate if they read zero when closed. Sometimes they are on sale for $2 or less.

BTW - It's just geometry.