Anybody running 17" Cobra Bullit rims on a Duster

hey if anyone could help me out on some info about the wheels and backspacing... and offset. im trying to figure this out and its all confusing me. 17x7 fronts on a 75 duster 0 offset and 4"backspace. where do you measure the backspace?the face of the hub in toward the spindle? the rears are 17x9 with 0 offset and 5"backspacing. i have the option of +12 offset on the 17x9 but that whell has a 5.47" backspace. how do you measure the offset to see where the wheels will end up? im not sure what exactly the offset even means. =offset sets the wheel out toward the fender more? or in toward the leafspring? any help would be great, thanks.