This is simple math use the formula I posted earlier or go to this link CALCULATOR
. You say that the car weighs 2400 on average 70 percent is up front 1680 or 840 per wheel so just calculate your spring compression use the calculator until you reach the magic number and you have it. This works for any normal a-frame setup, geometry only comes into effect on long a-arms like on a off road buggy or truck.
I would say design load would be up to the manufacture I do know that the main manufactures of springs like Eaton, Moog and others use a per spring rating not pairs.