For stock size tires, 7 degrees of caster angle is optimal. As the tire size gets bigger, the optimal caster angle decreases somewhat. Currie has found 4.5-5 degrees to be optimal for 35" tires with 5-6 degrees being about right for 33" tires.
So yours is a tad light but the exact caster angle is not critical. If after you complete a turn your steering has good "return to center" on its own, I wouldn't worry about it. If your steering doesn't return to center as well as you would like it to after a turn, then you may want to invest in either a cam bolt kit for your front lower control arms or adjustable length control arms... either of whch will allow you to adjust your caster angle.