I know I'm late to the party, but I thought I would provide an answer here for people who need to horizontally position an absolute item, when you don't know its exact width.
Try this:
// Horizontal example.
div#thing {
position: absolute;
left: 50%;
transform: translateX(-50%);
}
The same technique can also be applied, for when you might need vertical alignment, simply by adjusting the properties like so:
// Vertical example.
div#thing {
position: absolute;
top: 50%;
transform: translateY(-50%);
}