ThePhysicist talks about using the bits of information provided in the movie to learn about the basics of some of the physics behind it. That's why he points to the formula for calculating a Scharzschild radius and not the complicated solutions of Einstein's field equation for Kerr (rotating) black holes. As a learning experience, you don't want to dive into the hardest stuff right away. As a you learn, you'll also figure out what was a ball-park number and what wasn't.

Also, the effect is strong, but in all of the formulas you're already guessing factors such as mass of the black hole, which have a far bigger influence on the end result.