Никогда об этом не задумывался... Покопался в help
Есть такой слой BODY_CENTER, кажется он как раз для этих вещей и сделан.
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Origin
Specifies the point on each component to use for the anchor point. When you choose a component for placement, the program looks at the possible grid choices, selects the most appropriate, and aligns the specified origin with the intersection.
Choose from the following component origins:
Body Center
Defines the component center by adding a text point at that location. The text point must be of class PACKAGE GEOMETRY/PART GEOMETRY, subclass BODY_CENTER. If you do not define the component center on the symbol drawing, the system automatically uses PLACE_BOUND_TOP to calculate the body center and uses that point for placement.
Symbol Origin
Indicates that the origin of the component is (0,0) on the symbol drawing.
Pin 1
Indicates the pin that is defined as Pin 1 in the symbol drawing is positioned on a placement grid point.
If no pin is defined as Pin 1, the component is placed on the symbol origin.
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SYM_CENTER_X
SYM_CENTER_Y
The X (Y) coordinate, in user units, of the symbol’s body center. These fields are taken from the text point of text attached to the symbol with subclass BODY_CENTER. If that text does not exist, it is calculated from the PLACE_BOUND_TOP rectangle(s).