I personally think the percentage of classes at 50+ (or even 40+) is the more useful metric. Forgive me if I’m stating the obvious, but comparing the student to faculty ratios between universities and LACs is problematic.
For one thing, with universities, the numerator only includes a count of undergrads even when the denominator includes faculty that spend time with grad students.
For another thing, with universities the grad students also receive disproportionate focus. The CDS rules out faculty that only spend time on grad student work, but not faculty that spend some or (more likely) most of their time on grad student work.
Since LACs generally don’t have grad students, their ratios offer a more accurate description. Ideally there would be some sort of adjustment for universities based both on how many grad students the included profs are also teaching and how much more time each grad student receives individually when factoring in classes, advising, and research activity. I suspect USNWR has different rankings for LACs and universities partly because of such problems when comparing, but also because their missions are so different.
As a side note, there is a peculiarity with the Brown ratio calculation. According to their 2024-25 CDS, they have 1,022 full time instructional faculty and 57 part time. That would mean they have 1,041 full time faculty equivalents. Yet they use 1,238 in their ratio calculation. There’s also something funny with the student count, as they used the 7,199 in the ratio calculation despite section B indicating 7,191 full time students and 718 part time for 7,430 full time equivalents. If these are actual errors (maybe not?), then the ratio would be 7.1 to 1. But again that’s more usefully compared to other universities with similar undergraduate and graduate splits than to schools with no (or very few) grad students.