I like to use a very sharp pencil to trace around my inletting metal. Then I make my cuts to be all inside the pencil line by just a little bit.
Be careful about tapering the metal with a file too much to get something of a "wedge" shape. That might get you a nice tight inlet when you first install the metal, but as you file it down to get a nice flush fitment between the wood and metal, a gap will open up as you take the metal away (if you've made a straight sided inlet in the wood).
Try using soot from a candle, acetylene torch (no oxygen) or an oil lamp instead of inletting black. It doesn't smear and move around as much, leaving you false marks to chase. Alternatively, a sharpie can be used. It also doesn't "grind" in to the wood pores the way the greasy black does. If you still want to use inletting black use it very sparingly. Use a small scraper to take off the excess on parts you don't want blackened. That stuff has a bad habit of getting everywhere!
That inlet actually doesn't look so bad in the latest picture. Only the front part of the top line looks like it has an appreciable gap. Unfortunately, you won't be able t peen the plug in that spot. Remember, varnish is going to take up some space too. Once my tang is in final position (fully relaxed, with no spring to it when the tang bolt is tightened) I like to glass bed the tang to get 100% solid seating. If a little oozes in to that gap odds are you won't even see it on the final product.