How to get to % from a real number? It's not the distance from the left. Or from the right. It's the distance from the left to somewhere inside the inner bar. But where inside the inner bar? That's the number!
Awesome: cross origin requests are always fired, but cancelled if the response doesn't include a CORS header. The request is executed though! The server receives it in full!
If you hold off on the CORS header a few seconds, the request just takes a few seconds and then cancels or succeeds.