This is the perfect solution for wrapping for % and px, because of the grow features they can scale down and up. And even if you want to use the default 1140/1120 grid then it works.
This is a working solution for 12 column designs (in the 1140/1120 grid). So if in your design an element takes up 4 columns then it should be 32%. The other one grows automatically making it easy to make layouts on multiple rows.