When you do the ronchi test outside of ROC the lines expand (the grating is closer to the ROC so there are fewer lines/width) as you look towards the edge. As a result, the ROC of the edge is even further out than what the center is and that is called a turned down edge. Pretty typical problem on many of the first mirrors as the speed of polishing that beginners do tends to do a turned down edge. Depending upon how large the edge problem is, you may want to just ignore it and mask it off. I would consider up to 1/8" for a first mirror to be acceptable. Remember that you need to be able to improve upon what you have done on the next mirror! :^) Before trying to correct for the edge problem, take and do several sets of testing and see how repeatable you are. I've found that even with a TV camera to help, numbers like that are a bit variable and often getting a number that good is sometimes fudging the numbers (not that I'm saying you're intentionally doing it) but doing tests like this can be very subjective. You may find out that you really do have a 1/18 wave mirror and if the edge is small, just mask it off and enjoy a slightly smaller mirror than what you wanted but one that is excellent in shape. If the results are as you have said, you have done an excellent mirror on the first try. Now to do the superior mirror next time.