IMHO, that is why it is best to plan to start in Sarria proper, some 111 - 118 Km out from the Cathedral, depending on whose numbers you believe. If you start from Sarria and document it in your credencial, you will qualify for the Compostela (if that was part of your motivation). If you do not care about getting a Compostela, disregard the rest of this post...
Originally, there was a 111 Km mojone (granite marker) at the top of the famous Sarria stair steps, and around the corner, just after the local art museum. But, as of 2016, it was no longer there. I have a photo of the naked cement base from where it used to stand. It appears they used a concrete saw to cleanly sever the mojone, presumably so it could reused... It WOULD be the ultimate Camino souvenir...
More recently, while working at the Pilgrim Office this past summer, I discovered that the official "Cathedral" distance from Sarria is now listed in the Pilgrim Office computers as 118 Km, not 111 Km. When I asked about this, one staffer commented that they thought the distances had been recently reconfirmed using GPS technology, instead of a wheeled measuring device...whatever...
I suppose the point is that only the numbers maintained at the Cathedral office are official and binding. Relying on the current physical location of the "garden gnome" 100 Km mojone may not be accurate.
Parenthetically, over the past year or so, I have noted an increase in people seeking to access the Camino and start their pilgrimage EXACTLY at the 100 Km marker, not one meter farther out. To that effect, some of the posts in other forum threads have been either silly, amusing, or just sad, depending on one's perspective. I will refrain from offering further opinion on this phenomenon...after all, everyone does their own Camino. It is not for me to judge... But the focus on this point seems a bit out of perspective to me.
As this thread aptly suggests, that particular marker, the mysterious "official" 100 Km mojone seems to move about from year to year, like a garden gnome
. It seems that every named place or business along this stretch of the Camino desires to be "the definitive 100 Km" point. Relying solely on a "selfie" taken with that marker could wind up being disappointing if some cafe owner or just local pranksters moved the mojone a couple of Km just for yucks. Caveat Emptor folks...
So, my recommendation again is, to not shave this too fine. Start at Sarria and document your progress with at least two stamps daily until you reach Santiago. If you cannot manage the stairs, get a ride to the convent and church at the top of the hill, where you make the sharp left to head along a cemetery and out of town.
Remember, the Cathedral rules state that the final day's two sellos / stamps must be obtained BEFORE you enter the city of Santiago. Practically speaking, that is in Lavacolla, and likely near Monte de Gozo, slightly AFTER the Radio - TV Galicia station, at the cafe on the left (? Bar - Camping San Marcos - IIRC).
My advice is to obtain the stamps BEFORE you cross the footbridge over the Autovia into downtown Santiago. For those of you who have done this previously, you KNOW the bridge I am talking about. FYI, I read somewhere that it was being replaced and upgraded to something less scary.
I hope this helps.