I think the original idea behind using a Zone I density of .1
over film base plus fog, or to find the minimum exposure
necessary to produce a meaningful increase in density, was to
produce the thinnest (and therefore sharpest) possible negative
with the tonality desired, and to be able to use as much of the
density range as possible for N+ development before hitting the
Dmax.  If you get a Zone I density of .35, you adjust it by
increasing your film speed rating.  On the other hand, your
reason for testing for Zone III instead of Zone I seems
perfectly valid to me. The only thing you may lose by doing it
that way is the potential for N+ development if you place it too
high, and depending on the film, format, and enlargement plans
some small degree of sharpness, but that seems like a worthwhile
tradeoff, if it gives you the tonality you want for N
development.