Looking at diagrams of wheel measurements, it seems that backspacing is the distance from the inner rim of the wheel to the hub mounting point. This can be calculated by dividing the wheel's width in half (thus determining the centerline) and then adding (or subtracting) the wheel's offset.
With many of the wheels said to fit a 2012 Focus (5x108 bolt pattern, high positive offset), it seems their measurements are incorrect. For example:
Enkei Performance Ammodo
Bolt Pattern: 5-108
Rec. Tire Size:235/40-18
One or more of these measurements must be incorrect. An 8" wide wheel's centerline is 4", or 101.6 mm. Add the offset and you get 139.6 mm of backspacing, which is about 5.5". That's off by over half an inch.
These kinds of measurements are crucial on newer cars with ever-increasing factory offsets. Are these numbers supplied by the manufacturer?
EDIT: Did a bit more research and found that wheel width is not equal to overall wheel width, and 1" is a relatively safe amount to add to determine overall width. That means this particular wheel would have a centerline at approximately 4.5", and adding 38 mm of offset (1.496") would put you at 5.996" of backspacing. Looks to be more accurate, but to get 6.06" of backspacing this wheel's overall width would have to be 9.128".