Once you get it flat on some wet & dry you could just refresh with a smaller plate, but an 8 inch would be easier. I’m not one that believes that dead flat is an absolute, especially for a finisher, smooth is more important, but you should get it close to begin with and no dips. A lapping plate, almost half the size of the stone could be a problem, and just ride over the high spots.

Those stones are a bit on the brittle side, you may want to mount it or make a case so you don’t break it.

A 8 in DMT is not that pricy, about $50.