No, it does not have to be right on the original specs.
Offset, unlike backspace, is measured from the center of the wheel as zero, then the offset amount in millimeters (usually outward). So, if you have a 17X8 wheel with a 13mm offset it would have a backspace of 4.5118 (4inches +.5118inches). From there you can measure how deep/wide you can go.