Saggy springs

While changing the front lower ball joint I noticed the the springs are looking a worn and the car doesn't sit as high as other disco's. Once i had everything build back up I measured the height from the wheel arch to the floor and got 23" at the front and about 23.5" at the back. I have read the the front should be 19.7" from the centre of the wheel to the wheel arch.

Does anybody know what the height of the back springs should be or could measure their car and see what they get? I'm running 255/65/16 Grabber AT2's front and AT's on the back.