Card Column Preview - HTML and SCSS

Thanks to this challe i understand better how sizing works in css but there's still 1 issue that i'd like to solve:

• In this example each colored card has a certain width and some padding. When i want to specify a width with % i have to calculate manually the proportion of my desired width relative to the actual free space (that is card width - padding). In my css there are 2 cases (lines 138 and 167) in which i had to do that manually but i'm sure there's a smarter way to do that. I just want to specify width relative to the total width of the container, not the available width.