Check the App for the FP return times when you are on your way to the park to scan in. If FPs are still available on the App, you can buy MP once you enter and get one right then to use later. IF the App says they are out for the day, no need to buy MP if that was all you were buying it for. Save a little money since it will be a shorter park day anyway, arriving at 2pm.
**To complicate a little, sometimes the app will say all FPs are out for the day (and they are, at the paper machines), but if you already have bought MP, you can click on it and it MAY give you a FP time that is not available to the paper FP people.