With bootstrap 3 the best way to go about achieving what you want is ...with offsetting columns. Please see these examples for more detail:
http://getbootstrap.com/css/#grid-offsetting
In short, and without seeing your divs here's an example what might help, without using any custom classes. Just note how the "col-6" is used and how half of that is 3 ...so the "offset-3" is used. Splitting equally will allow the centered spacing you're going for:
your centered, floating column
