I was told by Carrea years ago, that the first 15 degrees didn't have too much affect in reality.
Here is something that used to be on The Aftco site
DETERMINE SPRING RATE
Ride rate correction chart when shock and/or springs are angled:
Mount Angle 10° 15° 20° 25° 30° Rate Multiplier .96 .93 .88 .82 .75
Example: A 200 # / inch spring mounted at a 20° angle gives a 176 # / inch rate when figured for ride rate (200 x 0.8.
SUGGESTED SPRING RATE
SUGGESTED SPRING HEIGHT
FRONT
REAR
Rate lbs. Total Weight on Front Wheels Rate lbs. Total Weight on Rear Wheels 200 600–800 lbs. 65 Under 800 lbs. 250 800–1000 lbs. 80 800–1000 lbs. 275 1000–1100 lbs. 95 1000-1100 lbs. 300 1100–1200 lbs. 110 1100–1200 lbs. 350 1200–1500 lbs. 125 1200–1300 lbs. 400 1500–1800 lbs. 150 1300–1400 lbs. 450 1800–2000 lbs. 175 1500–1700 lbs. 500 2000–2300 lbs. 200 1700–2300 lbs. 550 2300–2500 lbs.
650 2600 lbs. & up
DOUBLE ADJUSTABLE DRAG SHOCKS Stroke Suggested Spring Height 3" 7" 4" 7-8" 5" 10-12" 7" 14" 9" 14"
Here is Aftcos tech site.
http://www.afcoracing.com/tech_pages/dragshock.shtml