Hi, Dale,
This is when those of us who live outside western Europe really envy our European friends who can just make a return reservation as they get close to Santiago.
I've been doing this for 13 years and always agonize over how many days to plan for. The system I've come up with that seems to work is that I make a rough estimate using something similar to what Joe suggests in the previous post. Then I add 2 or 3 for rest days, and 4 for Finisterre-Muxia. The 4 for Finisterre-Muxia are extra padding that I can use if I find I need more time for my pre-Santiago camino. A few years ago, for instance, when I walked the Camino de Madrid and Invierno, I realized I needed some more time before Santiago, so I had to forego the camino to Finisterre but I was able to walk all the way to Santiago. I imagine it would be entirely frustrating to have to take a bus (if you didn't want to) in order to make up for unforeseen developments. I guess there's no way to protect yourself completely, but this four day cushion has worked for me.
It's always frustrating to see how many days we spend getting to our starting point and home from Santiago -- those are days I'd much rather be walking!