If I meter my usual way with the meter set for box speed, how is that not assessing box speed? It's not a quantitative measurement,
Yes, box speed as per ISO system IS a quantitative measurement, very well defined too...
en.wikipedia.org
Film speed is found from a plot of
optical density vs. log of exposure for the film, known as the
D–log
H curve or
Hurter–Driffield curve. There typically are five regions in the curve: the base + fog, the toe, the linear region, the shoulder, and the overexposed region. For
black-and-white negative film, the "speed point" m is the point on the curve where density exceeds the base + fog density by 0.1 when the negative is developed so that a point n where the log of exposure is 1.3 units greater than the exposure at point m has a density 0.8 greater than the density at point m. The exposure
Hm, in
lux-s, is that for point m when the specified contrast condition is satisfied. The ISO arithmetic speed is determined from:
S = 0.8 lx⋅s H m
This value is then rounded to the nearest standard speed in Table 1 of ISO 6:1993.
Without a densitometer the ISO speed calculation is IMPOSSIBLE.