As far as I know, it is the F with an octave keyI don't have an answer for you, but I do have a question.
I read in the MusicMedic balanced venting method (or maybe it was the Uppers, lowers and mids guide) that you should tune to "middle F" when beginning the process.
Is middle F the lowest F on the horn, or is it F played with the octave key?
Although, Musicmedic does split the keys into low (C2 and below), mids (lower stack) and uppers (upper stack), so maybe they do mean the F without octave key.
Hopefully somebody here can advise what is correct