Explanation
Divided by followers, by views or by reach, one post can produce results differing by an order of magnitude. Follower-based rates are a leftover from feeds that only showed your posts to people who followed you, which is not how any short-form platform works: these surfaces distribute to non-followers by default, so a video that travelled makes your engagement rate look terrible against a follower base it never needed.
Views is the honest denominator for short-form. Whichever one you pick, the rule that matters more is never comparing a number computed one way against a benchmark computed another, which is what almost every published benchmark invites you to do.
The common mistake
Leaving saves and sends out of the numerator. They are weighted far more heavily in distribution than likes are, and most engagement formulas ignore them entirely.