I have a couple of Math Questions regarding League

First of all "Hi, and thanks for being here!"

I have a couple of questions regarding Health. More specifically, why, and since when, people run 5 Armor and 4 Health Yellow Runes? Also, how do I calculate my Effective Health ( That means Actual Health + Armor/MR )? Is there a formula? And if so, can someone type it to me in this thread?

I hope you can help me with my questions and also hope this is not the wrong Subreddit!

Thanks you all :)