A stripped down version of 4563625 which is non specific to bootstrap and easier to understand the concept due to lack of clutter.

media-queries

@media (min-width: 1200px) {
  .makeMyStyles(940px, 15px)
  }
@media (max-width: 1199px) {
  .makeMyStyles(768px, 12px)
  }

.makeMyStyles(@width, @padding) {
  .makeHalfWidthColumn() { //  just an example mixin
    width: @width / 2l
    float: left;
  }

  .myContainer{width: @width; margin: 0 auto; }
  .myInnerContent{ padding:@padding; width: @width - (@padding * 2); }
  .myColumn { .makeHalfWidthColumn() }
}

//------------ CSS OUTPUT ------------//
@media (min-width: 1200px) {
  .myContainer {
    width: 940px;
    margin: 0 auto;
  }
  .myInnerContent {
    padding: 15px;
    width: 910px;
  }
  .myColumn {
    width: 470px;
    float: left;
  }
}
@media (max-width: 1199px) {
  .myContainer {
    width: 768px;
    margin: 0 auto;
  }
  .myInnerContent {
    padding: 12px;
    width: 744px;
  }
  .myColumn {
    width: 384px;
    float: left;
  }
}
//------------ CSS OUTPUT ------------//