'Usually' 1 coil = 2", but there's members here that have used that formula & attained more drop. Some are O.K. with that, others not so much.
Could be due to the different springs used by GM depending on the engine size & with or without A/C.
I would think that a V6 spring with 1 coil cut would yield a lower drop when installed under an SBC truck. Even more with a BBC. But I can't see anyone putting a V6 spring under a BBC truck.
I'd go 3/4's of a coil first and see how that works out. You can always cut more. Pretty hard to add-to if you cut a full coil & it turns out to be too much for your liking. Bit more work, but better safe than sorry.
Makes me wonder if cut BBC springs in my SBC truck would give a firmer ride. Hmmmm, might know a boneyard with a set.