I would ask the Bonsaihunk, Jerry Meislik. He wrote a book about Ficus.
http://www.bonsaihunk.us/
Furthermore I have to agree with jbhayman, deepen the wound as deep as you can, so the new callus can overgrow it much easier. I have Ficus Benjaminas with much larger wounds 9x9 inch (4x4 cm). I usually use knobcutters to get deeper in, but with a powertool carver, you can carve the wound deeper out as well, though you have to have steady hands.
Hope this helps, Wessel