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.