That is about the best way to do it, using Engis or Hyprez Diamond Paste.
Alternatively, you could ave it machined by Single Point Diamond Turning which will give an optical polish with extremely tight tolerances and physicaly stronger
This polishes beautifully to an optical finish with Hyprez or Engis Diamond Lapping compound. If you haven't done it before I'd sub- con it though, it's a skilled task.
Alternatively, Single Point Diamond Turning (well, flycutting for flats actualy) will get an optical finish too.
Hope this...
