When you put a chip in a piece of glass, it's wise to go back and use a grinding stone to clean it up and insure that there's no sharp edges. I'd use a 600 grit or finer stone (or you can use some valve grinding compound and a sliver of wood) and clean up the sharp edges. Since it's under the mounting stuff, it won't hurt anything even if it isn't ground smooth although you will have to take care when you disassemble the scope.
Twyman effects will cause the edge of the chip to stand proud by a little bit but that won't be big enough to cause any problems and since it's in the middle, again that will cause no problems.
When you do grind the edges, make sure that you don't scrape some grit or other stuff across the mirror's surface and make a bigger mess of things!!
Next time, take better care of the glass and don't drop things on it or drop the glass itself. Glass is really pretty sensitive to impacts as you have already found out.