There is no correct offset. It depends on what you want. Maximum back space is around 4 1/4". Almost any 7" rim will work from there.
It's sort of a personal preference like posted above... 3.25" to 4.25" will work pretty good and fit out back. Personally though, I like the tires tucked pretty good, so 4" is my preference. Tire size plays a bit of a role too... but not as much as the rim specs. Dave