During my research on new wheel/tire packages for my 76 Corvette, I came across some good info regarding offset and backspacing.
“Wheel back Spacing is very simple and is often confused with the offset. To determine a wheel’s offset you must first find the wheels back space. The back space can be measured in inches but should be converted to millimeters before proceeding to find the offset. To find the back space place a straight edge across the rear of the wheel and measure to the mount pad. The distance equals the wheels back space, which in this case is 6.75″. Multiply the back space by 25.4 to convert the back space to mm (6.75″ x 25.4mm = 171mm).
Wheel offset is the distance between the centerline of the wheel to the wheel mount pad. It is measured in millimeters and the outcome can result in a zero, positive or negative measurement. To have a zero offset, the wheel mount pad must be even with the centerline of the wheel. A positive offset means that the mount pad is shifted past the centerline towards the face of the wheel, while a negative offset means the mount pad is shifted past the centerline towards the rear, or the brake side of the wheel. To determine the offset of a wheel you must first measure the overall width of the wheel. Lets say the wheel measurements are 18″ x 9.5″. Now convert the width to millimeters by multiplying the width by 25.4mm (9.5″ x 25.4mm = 241mm). Next divide the wheel width by 2 to determine the wheels centerline (241mm/2 = 121mm). With the centerline and back spacing of the wheel determined and converted to millimeters simply subtract the centerline from the back spacing for the offset (171mm – 121mm = +50mm).”