I wouldn't worry about the actual number of degrees till you get some drive time. Any change to the ride hight can effect the caster. Mine change enough I had to change shims when I remover some leafs out of the front spring pack. It changed again when I lowered the rear axle. Both changes were only 1/2" to 5/8" lower.
Mine is at 4 degrees now and drives like a dream. You will know if it's to much if the steering wheel is hard to turn, you end up lifting the whole front end each time you turn. To little caster and they wonder bad and have zero road feel. At very slow speeds, little caster makes for very easy steering, plus you get no automatic return to straight ahead with to little caster.
