Rear Coil Spring height?

[Jarwood]

I need to pick your brains! I cut my front coils to get my height down to 29" from floor to center of my fender. I like the height for now in the front. I have not touched the rear but want my rear to be 29"-30". I do not want the rear of the truck to sit lower than the front. I need to buy rear coils. Here is the million dollar question. What size rear drop coils do I need to buy to get my truck at 29"-30" from floor to middle of fender? Ohyeah I have 20" wheels with 275/40 tires.
Thanks!