I would be surprised if D.C. Fontana would feel good about how her work was taken by the new Star Trek production... haven't heard any kudos from JJ or Orci/Kurtzman for DC... let alone any remuneration for basically lifting a couple of her scenes for the movie...
Screenwriting credits are arbitrated by the Screen Writers' Guild. The producers have no say on who gets credited. They simply hand in all existing script drafts, and the contributions are weighed up. The new movie pays homage to two brief scenes in "Yesteryear", but that's not enough that DC would be given a screen credit.
yes it takes more then a 50 percent contribution.