How do I make a absolute positioned div have a width equal to it’s parent minus some margin

Just specify all of the top, left, bottom, and right properties and the box will expand to be at all of those points.

.container {
  position: relative;
  width: 400px;
  height: 300px;
}

.inner {
  position: absolute;
  top: 0;
  left: 200px;
  bottom: 0;
  right: 0;
}

See the jsFiddle.