how much to cut off the spring for 6" drop in the rear

[kell490]

I want to just cut my rear springs down when they say 6" drop do they just cut off 6" of spring when its un compressed. I figured that wasn't the case but after you load the spring up the truck will drop 6". I would like to know if anyone has cut there spring and how much did they cut off I dont want to take them off again after I cut them. The truck is a 67 shortbed. I put the 2 1/2 spindles/ and 2" drop srpings in the front.