I agree that links are not always the best way to source your information. I have come across too many that no longer work. The familysearch citation is an excellent roadmap to find the census on ones own. It is not, however the same citation that Elizabeth Shown Mills has in her Evidence Explained. It contains the same info, plus more, and is in a different order. So if she was writing for this census it would look more like:
1850 U.S. census, Allen County, Iowa, population schedule, Perry Township, p. 532 (handwritten) dwelling 2736, family 2748, Charles Martin, NARA microfilm publication M432, roll 135, digital image,Family Search (http://www.familysearch.org : accessed 5 April 2016).
Its a bit more complicated and offers more info that may or may not be of value.