I don't know what the alkalinity or residual alkalinity of your water are but you can brew beer of any color from 5 (home brewers have a very difficult time getting below 5) to 90 with it. And how would you know what SRM you realized anyway? IOW the relationship between water chemistry and beer color, beyond the casual observation that darker beers tend to be made with water with higher RA, is tenuous at best. Worry about treating your water to get the right mash pH, not a particular RA.
Of course that bit of advice is, I hope, useful but it doesn't answer your question. The intent of the nomogram is to tell you what range of finished beer color you should be able to brew with a given RA or, turning it around, what RA you 'must' have for a finished beer of a given SRM.