I found a great web site to determine the 100 air mile radius from any point you select.

The webpage is called: Free Map Tools, Radius Around A Point

This is a copy of 100 air mile radius from Santa Rosa, CA

This is currently in Beta version, which can have a lot of bugs.   Not sure why the green coloration on the upper portion of map.

You can select your point of radius and then the miles, remember that 115.08 miles equals 100 air miles.

If you are trying to determine whether or not a specific location is within your 100 air miles, do the radius first then search for the location and if it pops up within the circle - then you are under the 100 air mile exemption rule.  If you have Google Earth installed on your computer then you can also output the radius - click on HERE for an example