[DanEC]

Roughly you can take the dimensions from the lower control arm inside pivot to the tire center, and the lower control arm inside pivot to the shock absorber amount and calculate the ratio of the two. For example if the ratio is 1.5 then that can be multiplied by the difference in spring length (1 inch) which all things being equal (spring rate and so forth) would drop the rear 1.5 inches.

It's not exact but should be reasonably close with accurate dimensions and ratio. Since the 8 inch spring has a slightly lower spring rate the actual drop should be a little more.