Hi, NC,
How exciting to be planning your first Camino. I think your question will elicit a lot of different approaches to this question.
I'm a big planner, so I always spend hours pouring over websites, blogs, forums, to come up with a reasonable day-by-day itinerary. I then add a few days for unexpected things and a couple of rest days, add on the travel time to and from the camino and buy my ticket. This is something that the Europeans on this forum don't have to worry about because they can just figure out how to get home when they arrive in Santiago -- if you're from North America, you have to buy a RT ticket, so you have to figure out how many days you can walk.
I don't make reservations ahead of time and I don't feel tied to my "itinerary" once I start walking. Things always change and I just don't want to have a rigid schedule. But I have some benchmarks -- e.g., want to be in Caceres by X date, want to be in Ourense by Y date, etc. So when I take stock of my progress I can tell whether I need to pick it up or slow down or what.
But the more difficult question is how you figure out how many kms you will plan on walking per day. I know what I'm comfortable with after all these years, but if it's your first camino, you aren't likely to know what your body will want to do for 750 or 800 kms. I think the average is about 25-28 kms a day, which divided by 25 is 32 days for St. Jean - Santiago.
Most people probably leave the albergue in the morning with an idea of where they want to spend the night, subject to the unexpected, of course! I don't think too many have a rigid day by day schedule set, at least that's not what I've seen.
Buen camino, Laurie