How to figure rear drop?

[Conundrum]

I was wondering how/if you can accurately guesstimate the amount of drop you will get with a rear flip kit or a homade rear flip for a 73-87 1/2 ton. I read once that you can measure the thickness of your leaf springs and then add 1.5" to that. But to me, I would think you would add 3" to that figure cause you would go off of the axle center line?.

Anyone know how to do this?

I made my own perches at work awhile back and would loike to figure out about what rear drop I can expect, so i'll know what exactly i've got to do for the front.