Basically you are effectively lifting half of the spring (assuming it is equal distances from eye to centering pin) so 2 in. shackles will give ya about 1 inch of lift.  I try and use shackles to correct for lost camber, or bad pinion angles.  For instance on my 84 I dropped the front spring hanger about 2.75 in.  With my 2 in. longer shackles I had about 0 degrees of caster!! I put 3 degree wedges in backwards to gain some of it back but I still havent got enough.  If memory serves toys like 5-6 degree's so I need to make some longer shackles.  Hope this helps
WT