It just depends on how much compression you want and the ride height you like. Do you have a solid axle or IRS ?
If it's a solid, I made a mock-up tool (or you can buy one) that you replace it with the coil overs for each side of the car. Two heim joints with a adjustable threaded rod. Once you have the ride height, use a floor jack to lift up the rear end. As it and the suspension raises, look to see if the tires will rub on the body, if not, then you're done. I also used the tool to adjust the coil overs so the weight of the car is about equal on each side. I was amazed just how off the weight was when I built the car !
If it rubs, you can either set the coil overs to stop it before that point, which it will bottom out, but you do want that, or depending how much it rubs, you can grind/sand some of the fiberglass off where the tire rubs. I had to take off 1/4" off mine, which doesn't hurt anything as you're only taking off the inner lip.
I now love my ride height and the suspension works very well.
