FYI, in a dual rate scenario, calculating spring rates is different:

Combined Spring Rate

This is used when two springs are stacked on top of each other.

Combined Spring Rate =
(Spring Rate 'A' x Spring rate 'B')
÷ (Spring Rate 'A'+Spring Rate 'B')

For example if the rate for spring 'A' is 200 and the rate for spring 'B' is 500, the combined rate is calculated as follows:

Combined Spring Rate =
(200x500) ÷ (200+500) = 143

(Source - Hyper Coil: http://www.hypercoils.com/TechTips/?TipNO=10 )

If this is correct then aren't you in essence running a soft spring? Isn't a longer 500 inch spring a better choice? I'm confused

 

Correct, until one of the springs is coil bound. Such as a helper, or tender spring. The rate goes to the spring that is not coil bound.

 

So it's 143# until totally compressed then the springrate shoots to 500#(unless totally compressed at ride height). I understand that the spring extends on rebound but on compression does it do anything functional?

 

It is compressed before you set the car down to ride height. It benefits you by keeping weight on the tire during extreme droop and by making sure your spring doesn't become unseated. It also helps by not "shocking" the tire so bad when recovering from extreme droop.

Other than that, it makes the car a hell of a lot easier to work on when having to use the jack.