Hey Dan,
This is how I worked out the rear spring rates on my car.
Car weight,
Total, 1500kg / 3300lbs
Rear, 860kg / 1900lbs (need to minus unsprung weight, about 300lb)
1900 minus 300 = 1600lbs. X .8 for spring angle = 1280lbs.
Divid that by 2 = 640lb, so with 400lb springs should move 1.6”
Then I set the Shock height on the axle and screwed the Shock spring to give desired preload and desired road height.
With that I have my AVO coil overs 14 clicks into 22 full clicks.
I haven’t changed weight into dynamic force, that’s where you need a professional.
Cheers Phil.