If you are curious like I was then simply remove the springs and use a jack and stands to raise the rear until you reach the minimum 13.5" eye-to-eye length for the shock. That will tell you roughly where your ride height will be. You may later end up needing shorter springs to get to that value but no need to worry about that too much until you get the weight in the car.
Yes, I just did this last weekend because I was curious
.
John