All good info, but will add that if you do happen to find RUC availability you need to insist that the agent immediately reissue the ticket to correctly show you in F. It will reissue into “O something.” Otherwise your ticket will still be Main Cabin despite having a seat in F. While this can be fixed at checkin by calling in then, the bigger issue is the inevitable schedule changes that will occur if this trip is far off. The computer will probably boot you back to main if the ticket isn’t reissued, and then it can be almost impossible to get back into F because the RUC inventory most likely will be gone.