I've got a nice set of 7 russian blades I got mostly from ebay( one from sham) that I got very cheaply to practice my honing skills on( the sham blade of course is perfect edge). Five have seriously dull but passable edges. Two have pretty serious chips in them

I put one on my DMT 325 to get the minor( but plentiful) rough edges out and it worked great til I discovered this half moon shaped divot in the center. How does one take the edge down to the lowest commmon flat surface so I can start over with a new bevel?

thanks