This is my first post and I actually only joined to get help with this. My brother-in-law is a flat earther and sent me this video about earth curvature.

Math is not my strong point and I feel a bit bamboozled by it.

I'd like to know if the math checks out on this video:

I also want to know if the math checks out at this earth curvature calculator - is it usable? https://earthcurvature.com/

Thank you. Please hold the ridicule and give me the math, baby.