I believe the spec is a measurement (of toe-in) from the centered position (no toe-in or toe-out) of the tires relative to the spindle (center). So, IF the spec is 1/8" toe-in, then the difference from front of tire to back of tire should be double that measurement or 1/4". So, if the spec is 1/16" toe-in, then 1/8" difference front to back is right.