I'm going to be a bit nerdy here. The mid-point of the
Camino Frances is at Terradillos de los Templarios very close to the crosshairs on this map:
http://www.mappingsupport.com/p/gmap4.php?ll=42.362476,-4.891453&z=18&t=h
The camino goes to Santiago on the road west from that point.
Actually that is off by just a tiny bit but that point is easy to find when you are on the ground in town. And yes, there are other ways to determine other points but this way is simple.
Here is how I got that point. The longitude of the bridge crossing the River Nive in St. Jean Pied de Port is -1.23724.
The longitude of the west entrance to the cathedral in Santiago is -8.545411. The mid-point is -4.8913255 and that crosses the camino at Terradillos de los Templarios at a spot very close to the cross roads at 42.362476,-4.891453
Santiago: 42.880528,-8.545411
St Jean: 43.16233,-1.23724