How is the exact distance between blocks calculated as a percentage?

Question

css

Hello! I always get confused with these margins and percentages)
I accidentally stumbled upon this site: https://elements.envato.com/graphic-templates/ux-a...
I don’t remember what I looked at in the code, but I noticed that the grid was calculated to the smallest share, i.e. block width and padding on the right: https://prnt.sc/jh5ef5
I'm wondering if they manually calculated this or what? )