back spacing on 20"

well its time for the knife and i was woundering how much backspacing i need to tuck in the inner fender on the bed (bed is getting tubed) and will the back spacing be differnt on the front ??

anny body?

i think that the back spacing is the same,i know a buddy has 20x8.5 with a +18mm offset and its perfect

thanks man i just dont want to touch my inner fender and burn the paint I seen that way to much