A maximum density
On this page11
The table
For a minimum type size of 1 millimeter, a maximum of 40 characters per inch. For 2 millimeters, 25. For 3 millimeters, 12.
Which closes the obvious workaround
A rule that specifies only a letter height can be met with a heavily condensed face, producing type that is technically tall enough and unreadable. A density ceiling stops it.
And it has a side effect this library cares about
A maximum characters per inch IS a width. So the regulation supplies the one number a packaging catalog can never supply.
Compare the topicals property directly
There, 21 CFR 740.2(a) gives a 1/16 inch floor and nothing about width, so the pages state the character count and the panel dimensions and refuse to compute a fit, because character width is a typeface property.
Here the fit is exact
283 characters at a maximum of 25 per inch is at least 11.32 inches of line. No typeface assumption, no invented constant, no estimate.
Same question, two regulations, one answerable
Which is the cleanest illustration in the library of why the drafting of a rule decides whether a catalog can help with it.
The density tightens as the type grows
40 per inch at 1 millimeter, 25 at 2, 12 at 3. So the allowance falls faster than the height rises.
Which inverts the usual intuition
A bigger container needs MORE inches of warning line, not fewer: 7.08 inches in band one, 11.32 in band two, 23.58 in band three.
And it is still easier on a bigger container
Because circumference and height grow faster than the requirement does. The absolute number rises and the difficulty falls.
It is a ceiling, not a target
Fewer characters per inch is always permitted. The number is the tightest you may set, which is the number to compute against.