For an 8" wide rim, you usually want to be between 4.5 and 4.75". Some cars can use a little more than that, some can't. Just depends on the car and the wheel/tire combo.
I say this every time someone asks so I guess I'll say it again. I would use this method to determine what wheels and what backspace is needed to fit on your car. It takes about a hour to do, but you will know EXACTLY what you need to fit your car.
That's a good start, but it's not as EXACT as it sounds. I don't have 1" of clearance to anything on my Duster. I only have about 3/8" to the springs, and like the car described in the link, mine only rubs on the inside of the wheel well when placed at extreme angles, like going through a drainage. I certainly don't have 1" to the quarters either, and I've never hit those. What I really need to do is add a 3mm spacer to the back and never rub on anything.
It's a good method, and using the dimensions outlined will surely get you a tire that fits. But it also leaves some meat on the table depending on what tires you run, how much sidewall they have, how stiff your suspension is, etc.
