Didn’t even know about this feature, but it’s brilliant. A quick idea of what you can do:

#bar {
  height: calc(10em + 3px); 
}

And regarding current support:

The calc() property for lengths is available now in Chrome 19 (Dev channel build) by use of the ‘-webkit-calc’ property, in Firefox since version 8 using the ‘-moz-calc’ property and in Internet Explorer since version 9 unprefixed.