Most of these things (absolute, fixed, relative and static layouts or padding/margin/border and the boxing model) I know. The problem (I think) is that I can't get the full picture.

As you say, I want to avoid tweaking things and using trial and error to fix things :/

Formally learning CSS as you propose is a path that is time-consuming but with high success probability so I may try it. Could you recommend me anything for that path? I'd really prefer reading instead of watching videos.