Pixel Border and Percentage width in Proportion

Question

I think I might already know the answer to this one but I need a sanity check!

Say I have

#gridtest{
width:590px;
}

I could change the

Answer 1

You could use CSS3 calc() function,

.selector{
  border: 5px solid black;
  width: -moz-calc(50% - 10px);
  width: -webkit-calc(50% - 10px);
  width: calc(50% - 10px);
}

SASS mixin

@mixin calc($property, $expression) {
  #{$property}: -moz-calc(#{$expression});
  #{$property}: -webkit-calc(#{$expression});
  #{$property}: calc(#{$expression});
}
article {
  border: 1px solid red;
  @include calc( width, '100% - 2px')
}