The spec from GM is that the lower control arm should be level with the ground when sitting at full weight on a level surface.
You can pull the spring out and put blocks under the control arms to get them into that location for eyeballing where you want the fenders to end up relative to the tires.
Then when you have your weight/distribution figured out you can figure out what you need to do spring-wise to get the steering geometry correct.
Edit: with the springs out, you could run a ratchet strap around the frame to hold the control arm instead of blocks. That would let you lift and lower the frame without needing to re-set the stands under the control arms.
