The PSE Centering Scale

Reproduced with permission.

Gem Four visually equal margins. Even after a careful examination, it will be impossible to pick a margin smaller than the other three.
Superb A Superb stamp will be extremely well centered, and it may take up to ten seconds to ascertain that the stamp is not perfectly centered. One side may be very slightly smaller than its opposite, though certainly not apparent unless the stamp is carefully studied.
XF-Superb A stamp of this grade will be very well centered, and at first glance will appear very nearly perfect. It will typically require a few seconds of examination to determine which margin is slightly smaller than the others.
Extremely Fine The stamp will be well centered, though a brief examination will reveal that the stamp is slightly off center. All four margins will be generous in size for the issue.
VF-XF A Very Fine to Extremely Fine stamp will appear slightly off center in one or two directions at first glance, but will unquestionably be better centered than most examples. The margins will be full with room to spare.
Very Fine A Very Fine stamp will be off center on one or two sides, but the frame lines will not be close to the edge on any side. All four margins will be unquestionably full, and well clear of the perforations.
Fine-Very Fine The stamp is noticeably off center with one or two sides significantly closer to the edge than the opposite side. But there is no question that there is white space between the frame line and the perforations.
Fine A stamp that is quite off center on one or two sides with the edge of the design approaching the perforations. While close, the frame line cannot be touching the perfs. Some white space must be visible, though it may be very narrow
Very Good The edge of the frame line just touches the perfs on one side, but the perfs do not cut into the design.
Good The perforations cut into the design, and some portion of the design is lost.