The "old" general rule was 1/2 the fan blade depth has to be outside of the fan shroud. The motor side of the shroud should be at, or close to, the center line of the spinning fan. More or less fan blade inside the shroud was considered as not as efficient. The fan blade completely inside the shroud tends to block air flow at higher vehicle speeds, and a fan completely outside of the shroud allows too much air to escape around the fan blades and doesn't pull as much air during low air flow conditions.

Like many things these days, who knows if that is considered correct anymore.