One of the tools we can use to better evaluate our relationship with a DNA match is the total amount of shared DNA.
The International Society of Genetic genealogy has a very nice, though somewhat generic, table of expected values. You can find that here: http://www.isogg.org/wiki/Autosomal_DNA_statistics.
The power of crowdsourcing genetic genealogy is truly manifest in the following two efforts to gather and report the total amount of shared centimorgans (cM) between individuals with proven relationships. You can use these tables as a guide for your own research.
From the genetic genealogy community we also received a table on how to better use the numbers coming out of AncestryDNA ('cause they are different).