Hi again acorngen,
The speaker on Saturday did point out the restrictions on the A-DNA matching and it is clearly stated on the website with the percentage match levels that would indicate each level of relationship and the corresponding confidence levels.
However, the problem we want to solve is that we have documentary evidence that indicate quite a high likelihood that the other researcher and I are third cousins. Our great-grandparents had the same family name, arrived in the same place in England, from Ireland at about the same time, and on two different censuses were found living as close neighbours and their names appear as godparents for each other's children. The only thing we don't have is any document saying that they were actually related.
I have traced all 8 great grandparents, all with distinct family names. So any third cousin should appear on my list as such. According to what I understand of the test methodology, if any closer relationship was indicated, it should only be because one of them has a double-dose of genes from the same family further back (but not much further or too diluted to make much difference, so perhaps one or two generations further back). If a relationship, but more distant, was indicated then, the two potential siblings would be cousins at closest and would then be towards the limit at which this test could confirm what particular relationship.
However, there is no reason to suppose that we have any other lines in common, so if there is a relationship strong enough to show up on this test, then it is only likely to be on this one line.
Of course, to demonstrate that the test parameters quoted by the company are correct, it would be nice to see the results from a sibling and one or two first cousins, and some second cousins across a variety of lines, and other third cousins too for which I do have full documentary evidence, but that's wishful thinking, though I might suggest it to a few others among my contacts, if we appear to get a result here.