A lot depends on the relative stiffness of the front and rear suspension and the vehicle's weight distribution:
- http://www.4crawler.com/4x4/ForSale/Shackles.shtml#FAQ4

Yes, simple geometry would say lift = front-length/full-length * shackle-length, modeling the spring as a simple rigid member. But, like the way they adjust race car weight bias by "jacking" one corner or the other, the longer shackles can do the same as well as changing the load on the front and rear half of the srping pack itself. I have seen anywhere from 0" lift from a 1.5" longer shackle to 2" of lift from a 3" longer shackle. So if you are looking for exactly X" of lift, a shackle is not a good way to go, since it is so hard to predict the exact lift a given shackle length will provide.