Empirical formula of Berthollides
"Berthollide" seems to be a word for "non-stoichiometric compound" that has fallen out of use.

You have to look at the structure, as a rule things happen for a reason.  The formula can be derived from the structure.

Consider the series of alums, which is  KAl(SO4)2·12H2O, or more generic:  X+Y3+(SO4)2·12H2O.  The generic structure can accomodate a monovalent cation, provided it's sufficiently large (lithium is out of the question, sodium is an edge case), and a tervalent cation (Al, Cr, Fe, pretty much anything), and the proportion can be freely chosen within the constraints of the structure.