The problem with the steel dump method is that you first have to have a surface with a good spherical shape in order to glue them to the surface of the tool so that you can get a good contact in a nice sphere. The tools usually don't do well until several mirrors have been ground on the tool. This corresponds well with it's predecessor in design, the cast iron tool which is machined on a lathe to the right curvature and then worked on a number of pieces of glass, getting a better shape as it goes along.
Better to dig the hole with a ring tool and, after a rough spherical shape is gotten, cast a tile faced tool and grind from there for a one off mirror.
I might also note that you really can't use a steel dump (or a cast iron tool for that matter) for doing hogging as you will then indeed change it's shape by a little bit (things do wear!) and the rest of the surface needs to conform to this new shape.