Once you know it, it’s fairly simple. If I remember correctly, if you’re staying on site, you get 2 months to book your FP and if you’re not staying on site, you get 1 month and basically get the scraps that are left.
Add in the 6 months you get for dining reservations for staying on site and it’s the same issue.
The real problem is that you lose all flexibility. Every day of your trip is completely locked into one park and once you use your FP, your options for the rest of the day are limited and if you can only get a FP for later in the day, you’re screwed.