The volume bar is 5em, so 4.5em/5.0em = 0.9, so I don't think there should be any difference in placement? On one side it feels a little weird that JS would then switch to percentages, but the percentage doesn't directly map to the volume percentage anyway, since we have to account for the handle width. Using ems makes the mental math easier and would make it possible to do some auto-calculations with .less vars. So that's the way I'd lean if you're cool with it.