I'm hoping to pull on everybody's wisdom here. I am in the process of flattening the backs of some vintage Stanley 750's. My question is; What is flat enough? After 220 grit sand paper they have an almost mirror like finish, I was planning on going to 400 and stopping. When I lay the ruler from my Starrett 6" square they are not flat and there is light showing under the ruler. Is this something I need to be concerned about or not? The base I am using is a piece of certified marble for machinist work I acquired. Any thoughts or guidance?