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OK - given the lack of real "raw" numbers (i.e. exactly how many whites, blacks, Hispanics, etc. actually attend mass in a parish), your method would yield some interesting answers - but not entirely accurate ones. This is not your fault, or anyone else's. One must realize that census data is far from being 100% accurate for the simple reason that everybody does not answer the census, much less honestly. If I were a family of illegal aliens I would probably not answer it out of fear - many do this. Some do not give truthful responses because they resent the increasingly complicated and invasive questions asked. Heck - I personally never got a form in the last two censuses!!! Nobody came to my door either!
Also we still have the issue of unregistered parishioners. I have been one of those, and I know I am not alone..."known but to God"! Also, I am one of many Catholics who does not live in the boundaries of the church I attend mass at. And I am certainly not alone in that!
However, I accept what you say as a way - flawed though it may be - of determining the ethnic breakdown of parish populations, as these methods you raise are the best available.

No method is perfect, but a large enough sample will give a good indication.

BTW, when I lived in NYC, I lived on W. 74th St., but was registered at Corpus Christi up by Columbia on 122nd St.