Probably as far from a pro as anyone here, but I 'revived' a few chains recently with a $30 grinder from RK. They wouldn't cut at all and now work fairly decent again. They make a pretty good pile of chips beside a Hickory log. I don't take care of things well and they probably hit the dirt more than a few times, or went through mud on bark. Lengths from a 10" pole saw chain to a few 18s. First couple I did, it seemed to take too much off, so I had to make some adjustments and learn what not to do. It's not a great tool, but it serves my yardowner needs.
I tried some of those diamond grinder bits for a Dremel tool. Wasn't wild about them.
When I use a file, I just try to 'feel' for smooth. When I first put the file in, I can feel and hear a rough edge. Just a couple of strokes and I can feel the difference. I don't try to take a lot off that way.
I've cut Locust, Red Oak, Maple, Cherry, Sassafras, Cedar and Hickory so far plus a few I don't really know what they are. Some were cutting live trees, others were deadfalls or stuff I had in the pile for a year or two. Locust gets like concrete after it sits in the Sun that long.