it isn't always straightforward..
What folk put as the starting point on the form isn't critical as the staff can manipulate the details and the distance on the database after looking through the credencial and chatting with them (though of course, and with some thought, you should be as accurate as you can). In this particular case, if you the pilgrim wanted a distance cert, and we established there were just one or two short breaks, I would put SJPDP as the starting point and subtract the couple of bus trips. But if it looked like you were on and off buses all across Spain with lots of missing stamps but started walking in earnest from Sarria, then I'd put Sarria, plain and simple, 115km. Once the certs are printed, that information gets logged automatically as a statistic. In other words, nobody is a statistic until their certs are printed.
The online form can be confusing and you can't always put what you want, but if you try sometimes, you might find, you get what you need..