Btw, since it's been mentioned, stereo vision (two eyes) is required for depth perception (as is "stereo" hearing for aural direction sense).Seeing as I'm missing an eye, I have to agree with the estimation to limit missile fire. It *really* screws up your depth perception. The spot penalty would likely reflect on the lack of peripheral (However you spell that) vision [slightly edited]

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.