LayoutUnit offset = percentageOffset * availableExtent;
with the following test case
<input type=range value=75 style="-webkit-transform: rotate(30deg)">
produces an off by one result when subpixel is on.
Created attachment 221289 [details]
slider knob position
The new position does not make the slider's knob look less centered. It is just a different position. Prefer subpixel layout. (Can't compare to FF as this is a custom control)