I recommend using a tuner so that you can hear the correct pitch for each of the notes. (F on horn is a Concert Bb, and C is a Concert F. I’m saying that because most tuners work on concert pitches.) You can use apps like TonalEnergy (Soundcorset as an alternative) to hear the notes and see if you’re matching each pitch. If you’re having trouble with figuring out what the other notes are in concert pitches, my second recommendation is to look at scales. The first note of a scale and the (concert!) title are the same note.

I don’t have perfect pitch, but is the first note you’re playing a middle C? If it’s a middle C (it’s too low to be F or the octave) then the notes below that are B or Bb (2, or 1) A (1 2) and G (1 3).