There are two settings which may be a bit confusing, so here is some clarity that also will serve as a reference you can come back to.
If you Google for instance “15 miles equals how many kilometers?“, you will get the result “24.1402” But when you enter this, it calculated as ” Radius: 24.14 kilometers / 14.967 miles”. This is not quite 15 miles. There seems to be a calculation discrepancy in the equation somehow, but rather than spending any more time on this, if you want your maps to display miles without any decimals, use the numbers below.
Radius: 8.06 kilometers / 5 miles
Radius 9.68 kilometers / 6 miles
Radius: 24.19 kilometers / 15 miles
Radius: 48.39 kilometers / 30 miles
The lower the number, the more zoomed out you become.
So to display the following radius best set the Zoom Levels as follows:
5 miles = 11.675 zoom level
6 miles = 11 zoom level
15 miles = 9.87 zoom level
30 miles = 9 zoom level