Lwb to swb...6"or 8" cut in rear???

[hartleymetalworks]

I followed rokcrln instructions on this site (yesterday Actually)and cut 8" from rear and 12" from the front. But today i read a build on street trucks where they only took 6" from rear. If a lwb is 8' and swb 6'-6". Wouldnt the cut be 6" be correct????. Maybe i misunderstand somthing because ive seen dozens of people cut 12"and 8".... Then again maybe its wrong! Help