The rankness of a fart can be expressed in decibel-Farts (dBF), a logarithmic scale where dBF = 10 log10(F1/F2).
Characterizing F in standard units is a bit of a chore, but broadly speaking the resolution concentration of hydrogen sulfide (H2S) in air for humans-- the concentration at which 50% of people can detect its odor-- is about 4.73ppb, or 4.73 x 10^-9.
A cubic meter of air at sea level (1 atm or 760mmHg) at 15°C contains about 2.53 x 10^25 molecules of mixed gasses. Scaling to a cubic meter, the volume for detection of H2S for humans would be about 1.19 x 10^17 H2S molecules per cubic meter. This value is your F2, our reference intensity for detecting farts.
A typical human fart has a concentration of anywhere between .001ppm and 1ppm of H2S concentration, or between 1x10^-9 and 1x10^-6, or scaled to between 2.53x10^19 and 2.53x10^22 H2S molecules in a cubic meter of air.
Therefore the rankness of a typical fart could be expressed as a decibel ratio vs the resolution density falling somewhere between 23dBF and 53dBF.
This is a useful expression of rankness for modeling attenuation over time, accounting for dispersion, wind drift, and distance from the zero point.