My advice is get a calendar (I just printed out some pages from iCalendar!) and literally plot day to day. The only guess work should be how long the Camino takes. For most people, it's between about 32-37 days from SJPP to Santiago. Leave a few days extra incase you find yourself sick or injured.
I ended up changing my flight for this upcoming Camino because I panicked about the price of my flight going up and didn't really sit down and plot it out properly before I booked. My flight change was $100 but it was because it was a student fare, and I had a very limited number of different days I could change it to, but luckily it worked out. I added 6 days on to allow for the unpredictable. For me, I'd rather have more time and possibly ending up having a few days to kill than to be rushed, so I left more cushion time. For example, I want to walk to Finisterre, but may not be able to walk it; last time I had severe tendonitis and therefore knew I shouldn't attempt it. So I had to make the judgment call of whether to leave the extra time incase I can walk it and risk having more days than I need, or to not leave that time. You'll have to make the same sort of decisions for yourself.
Good luck!
Lindsey