The difference between the tap size and the hole size is your thread depth. The ideal depth varies depending on the thread pitch. With a 6.6mm hole, you have a .2mm thread depth which, of course, is correct for a .5mm pitch. You can go smaller on the drill by a few tenths if you want, but don't go bigger. With a softer material like brass, you can get away with it. With something like stainless, you don't want to go any deeper on thread depth than you need to or the tap could bind.