It's guess and check. There is not a linear relation between spring height and final stance height because it depends on spring rate and weight of vehicle. Basically you cut half a coil, install and see how much it changed, then make a judgement call. Cut another 1/4, 1/2 coil or whatever you think. Just go slow, you can always cut more, you can't add back.
Keep the springs cool! Don't get them hot or you will ruin their integrity. Use a cut off wheel or sawzall, not a torch. Cut slow and feel the spring if it gets too hot to touch you need to stop and wait for it to cool. Also don't cool it rapidly by splashing it with water or anything like that.
Keep in mind, I would not recommend cutting any of the ram springs except the progressive ones (which as far as I know are only the RT rears).