Responsive Single Price Grid Component - BEM, flexbox, media query

I'm not subscribed to pro, so I want to know how can I get the measurements right? I've been doing it trial and error, and it takes up a lot of my time. Can't seem to get the exact measurements no matter how hard I try.

And can I get any advice in using BEM? And if there are still things I can improve on, please let me know. Thank you!

P.S. Haven't applied shadows. I'll update this soon :)