The correct way to do this is to use the progress
callback function which jquery provides. See the docs: http://api.jquery.com/animate/, and within that, use only the parameters it gives you;
$thing.animate({left:100},{progress:function(animation, progress, remaining){
// do stuff with progress
}});
(Edited: sorry, progress not step)
Here is an update to your fiddle: http://jsfiddle.net/a9eXE/78/ (I actually am using step there, simply because it's easier to get the current tweened progress, and it makes no difference when you're only animating a single property)