There is a lot of great information about using DNA to triangulate matches and do genetic genealogy. Some very good information if you take the time to read it, which a lot of folks do and some don't. Some don't get it, no matter how hard they try. What's a cM, what's a valid match, how much means what degree of relationship? Questions.
What I have learned from all of the tests I have had done now is this. Read everything. Get a good grasp on what triangulation is. The difference between an X match and a match elsewhere. Use the tools out there, but take the "estimates" of what DNA means what relationship with a hefty grain of salt.
As I have mentioned before, my mom has a 74 cM match on chromosome 11. At this point I have lost count with all the folks who match her there, but I can tell you that everyone I have been able to trace, has had one shared family group other than one in mom's tree. And that family group, would be her 6th great grandparents. Yes 6th, and she is not closer related to anyone than 4th cousin 2x removed and averages 24-35 cM matches with most of over 30 matches.
And then after I tested my great Aunt, and my daughter, I got to see a few more interesting things. My mother, her Aunt, myself and my daughter match a family who I already had "figured" out was on my maternal grandmother's side, just not how. Here's the kicker though, we all match at about the same amount of cM's (in the same spot). Four generations with what amounts to a "third" cousin match in total cM's. But to find out how we relate, I have to go back to my great Aunt as a potential third cousin, because four generations are sharing the same amount of DNA.
It's also been interesting to see how much and how little of DNA from various family groups has passed on from one of my parents through me to my daughter. I wonder if it has anything to do with the function of that DNA, and you know the stuff we learned back in the dark ages about dominant and recessive, or is it purely just random.
Not a very informational post I know, but my point is this. You should test your DNA. You should learn what it means, and for christ sakes, don't just do it for the ancestry composition alone. You should educate yourself as much as possible, and then realize, that the science is still not there a hundred percent. The "you should" have this amount of DNA once you get past your grandparents kind of goes out the window, and because as I saw with my daughter, she didn't get equal amounts of DNA from both my parents, it will affect every generation you go past. Sticky DNA isn't a very scientific term, but it does explain why some pieces of DNA don't seem to go away. So when you look at that match of 30-40 cM just realize, you may have to dig further back then you realize.