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).