Processing signals with phantom fundamentals
Methods for estimating a fundamental pitch frequency (missing or not) from an FFT result containing a series of harmonics include finding maxima in a cepstrum, the harmonic product  spectrum method, and looking for clusters in histograms of the distances between frequency pairs of higher order frequency peaks that fit within psycho-acoustic filter bandwidths..  These are all more estimation algorithms that post-process an FFT result, rather than a simple formula.