Here is a quick method for calculating 100km distance from your home address.
Go to the website geoportail.gouv.fr
Click “cartes” at the top left of the screen and then choose “Plan IGN”
Enter your address in the address box and click OK to bring up the next screen
On that screen click the blue spanner at the top right side of the screen. Then click the “mesures” option
Click “calculer une isochrone”
Put your address in the box market “depart”
Click the “isodistance” option and enter 100km in the distance field
Click “calculer” at the bottom
The map will highlight a 100km radius of your address – you can enlarge it to get to the detail using the +/- box at the top right corner of the map.