Just to add to the discussion: the citation for the actual census record and the citation of the source (Ancestry.com) where you found it must be cited. Further, there is one professional opinion (Elizabeth Shown Mills) that not all NARA records have a common citation. Some have page numbers stamped, some do not. So, the source of the information is the NARA record, and your source for it is Ancestry.com. Ancestry is a secondary source and, unless you've actually viewed the original NARA record, Ancestry has to be cited along with the NARA record. I recommend Mills's book "Evidence: Citation and Analysis for the Family Historian."