offset on wheels to tuc with a step side

So i am ony rollin on 17's and am going to be doing 20's. what offset and or back spacing is everyone running. my truck is a step side so it has a lil more trouble. oh and how wide as well as the offset.

I've got 20x10s with a around a 40mm offset I believe.

well i am looking at a couple wheels that are 8.5" wide and they have a +38 offset. i just don't want them to sit in too far or rub on the step side bed. as far as my meaurements they should tuck just wanna see what every one else is runnin