Too general question.
Depends on the following factors:
* range of the F. Hrn. Too low? Too high?
* what the other 4 instruments are doing. If you have a flute, switched to a picollo, at fff, then... well... go for fff and open bells
etc...
* lines, melodic material. If they are playing different things, then same, if they are doubling, maybe lower, depends on the range.
* open or close.
Usually (51%) the F. Hrn is playing with the hand inside the bell, which results in somewhat more tamed sound, so...
* a mute maybe? (not the most often met thing with a F. Hrn, as compaired to the rest of the brass)
* anything else you can think of about
your own score and music
Sorry I can't be of any more help!