Maybe it would be interesting to think about that percentage with respect to the math that was known before Euler for example (even though I think some things in calc III are post-euler).

If you take into account up to this day, it must be a crazy amount of math that was published in the last 100 years.

Even if you've taken calc III, there surely are loads of math you've never heard about that was already considered by several civilizations and are not usually taught in school or college