But there's frequently more than one "typical use,"

Generally, yes, often depending on the region and other context variables. But not in the case of "folk song": there is only one principal meaning in the whole English-speaking world, similar to words like "tree" (what is the minimal size?) or "hut" (how small/fragile?).

I doubt anyone has done this or would want to

So do I - it is only meant as an illustration of fuzziness (see "Fuzzy logic" on Wiki).

What is typical usage? Often there is no way of proving it, we have to consult those whom we trust to be experts, e.g. the authors of respectable encyclopedias. Some words are used by the majority of speakers in ways that are disapproved of by a strong minority for good reasons; the use by the latter can still be considered "typical" until they are too small in number. No such problems with "folk" though.