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To add to what Larry said, do the same for each of the corners of the bounding rectangle.  That will get you closer than just locating the center. 

Also, if there are any obvious straight edges in the original, and the new object, lay down some tilted guide lines on a temporary layer to mark out the angles they are at.  That will get you even closer.

Another method is to put the new object in its own layer, reduce the opacity to about 50%, and then free transform it by eye so that it is in best registry with the object its replacing.  This isn't a very technical approach, but it works quite well in practice.

T