Okay guys, so I have the rules, I click on 2 points and I get the coordonates for that distance. It's not what I'm looking for. How can I check the width of the selected distance (with the ruler, or whatever method) in px, so I have an exact idea on my css writings. Thank you.
Ctrl+click that element and check its dimensions on the Info Panel (you have to switch from useless Navigator one).