Center Triangle at Bottom of Div

Can't you just set left to 50% and then have margin-left set to -25px to account for it's width: http://jsfiddle.net/9AbYc/

.hero:after {
    content:'';
    position: absolute;
    top: 100%;
    left: 50%;
    margin-left: -50px;
    width: 0;
    height: 0;
    border-top: solid 50px #e15915;
    border-left: solid 50px transparent;
    border-right: solid 50px transparent;
}

or if you needed a variable width you could use: http://jsfiddle.net/9AbYc/1/

.hero:after {
    content:'';
    position: absolute;
    top: 100%;
    left: 0;
    right: 0;
    margin: 0 auto;
    width: 0;
    height: 0;
    border-top: solid 50px #e15915;
    border-left: solid 50px transparent;
    border-right: solid 50px transparent;
}

You can use following css to make an element middle aligned styled with position: absolute:

.element {
  transform: translateX(-50%);
  position: absolute;
  left: 50%;
}

With CSS having only left: 50% we will have following effect:

While combining left: 50% with transform: translate(-50%) we will have following:

.hero { 	
  background-color: #e15915;
  position: relative;
  height: 320px;
  width: 100%;
}


.hero:after {
  border-right: solid 50px transparent;
  border-left: solid 50px transparent;
  border-top: solid 50px #e15915;
  transform: translateX(-50%);
  position: absolute;
  z-index: -1;
  content: '';
  top: 100%;
  left: 50%;
  height: 0;
  width: 0;
}

<div class="hero">

</div>