Reticle Subtension Math Apps. and Adaptations of Formulas

See a lot of posts about applying reticles for downrange zeroing and rangefinding, so I thought I'd reference this for anybody who might be interested. Sorry about the poor quality, I'm a camera novice for sure--

https://www.youtube.com/watch?v=iNvJKBOpj08

https://www.youtube.com/watch?v=ozEhoNaRi2s