Some things from published sources that may help:
The Quintessential Fighter has rules about called shots to the eye. A creature subjected to a successful one is subject to a -4 circumstance penalty to attack rolls and reflex saves. They also suffer -4 to all dexterity based checks, and -10 to movement. These penalty's don't seem very balanced or logical to me, but YMMV.
The SRD has some psionic effects that work in the opposite direction:
Expanded Vision:
...
The manifester is granted a 315-degree arc of sight, allowing him or her some slight vision of creatures that might otherwise fully flank the character. Opponents flanking the character gain only a +1 bonus on their attack rolls instead of +2 (although rogues can still take sneak attacks). All the character’s Spot checks gain a +1 enhancement bonus. Concurrently, the character suffers a –2 enhancement penalty against all gaze attacks while the power persists.
and
Ubiquitous Vision:
...
This power grants the manifester reduced penalties and bonuses in specific situations. In effect, the manifester has a 360-degree sphere of sight, allowing him or her perfect view of creatures that might otherwise flank the manifester. Thus, flanking opponents gain no bonus on their attack rolls instead of +2, and rogues are denied their sneak attack because the character does not lose his or her bonus to Dexterity (but they may still sneak attack if the character is caught flat-footed). The character’s Spot checks gain a +3 enhancement bonus, and his or her Search checks gain a +1 enhancement bonus. Concurrently, the character suffers a –4 enhancement penalty on saves against all gaze attacks during the power’s duration.
If you were to extrapolate these effects, a person with only one eye would have a -1 to spot and search checks, would give flanking opponents a +3 instead of +2, and would get a +2 enhancement bonus on saves against gaze attacks.
If you wanted to play up the limited field of vision more, you could rule that enemies only need to be at 90 degree angles to each other (rather than across from each other) in order to flank.