where U is the sum of the total number of use citations by all informants for a given species, divided by the total number of informants (ns).
The fidelity level (Fl),
FI=Np/(N×100)
where Np is the number of informants that reported a use of a plant species to treat a particular disease, and N is the number of informants that used the plants as a medicine to treat any given disease.
This should help you: Andrade-Cetto, Adolfo, and Michael Heinrich. "From the field into the lab: useful approaches to selecting species based on local knowledge." Frontiers in pharmacology 2 (2011): 20.