I like this item. I believe the radius should be 60-foot and the cone should be 30-foot. It operates like a 360 degree gaze attack. If the user holds the eye out with one hand, his hand is blocking the rear vision, creating a forward cone.
You may wish to set a duration on how long the eye stays open, and whether the eye stays open if the user no longer has possession. It could be very interesting to roll this thing down a crowded street. ;)