According to an In-Fisherman mag article : " The forward and upward position of the eyes on a crappie indicate that crappies see best and strike most accurately when positioned slightly below their targets. Crappies orient themselves to prospective forage much as bluegills do, turning and aligning themselves with the target, and approaching it until it's just above their snout."
The "strike zone" can be a few inches or several feet, but depends mainly on water conditions & the mood of the fish. I've seen Crappie on cover come up 2-3ft off the top of that cover, just to inspect or intercept my jig. Crappie do tend to "look over" their prey before deciding on eating it or just swimming away. The exception to that is when a jig or crankbait is being dragged thru the water at 1-2mph, and then it's mostly a "reaction" bite or they're actively following & feeding on baitfish schools and your bait comes into that "zone".