I have this very basic layout:


How is the best way to do it so everything is aligned correctly? I want boxes to match up on both the left and right edges and with a % margin between them. I also want this to happen at anything as low as 300px wide and up to 190px wide. If you adjust the width of my example it's all over the place.

I'm guessing this is simple, but I can't get the hang of it.