Speaking of eyes, we realize one reason the contract mark might be on Ciel's eye during the manga's circus arc. Right before Ciel summons Sebastian for the first time while in the cult, a member stabs him in the eye. You have to look close to realize that's what he stabbed, but, indeed it is. Perhaps putting the mark there also regenerates the missing part, as well as provides for better tracking?
If this is so, and also applies to the anime, by the end of Black Butler II Ciel Phantomhive has 'died' around three times.