Bust Of Lincoln

Centering seems to be of the utmost importance ,, on a stamp like this one where there are no defined boarders does it then become a center from the device on the stamp ?

Scott #1114 Bust of lincoln

Here is how I would do it,

The edge of the design at the top is defined by the top of the A and the L in the President's signature.
On the left it is defined by the left edge of the U in United.
On the bottom it is the bottom edge of the letters in UNITED STATES POSTAGE.
On the right the edge of the design is defined by the edge of the teeth in the final E in POSTAGE and the right edge of the C in 3c.

Now you can see the invisible box which contains the design. The centering of this box is important. Looks like a VF.