I created the chart below as a reminder to myself the level of effort and/or difficulty in trying to track down a match based on your shared CM.
I typically don't waste my time on anything below 15CM, and try to stay around 40CM or higher. I use 40CM as a guide because I can easily remember 40cm is roughly the range of 4th cousin. So 20cm is around 5th cousin, 80cm is around 3C, etc etc.
Anyway, as CM halves each generation going back, the number of possible ancestors the DNA came from doubles. Using 30-year generations, the ability to distinguish specific Ancestral DNA (IBD) from the growing influence of pedigree collapse (IBS) becomes difficult after six generations, and near impossible after 8 generations.
So when someone asks why I don't use cm values below 15cm (or small segments below 7cm), I typically respond with, to what purpose? Me and someone may have a 10th GGF in common. We also may share a 4cm segment. The two facts can't ever be tied together, even if we find someone else who shares the same ancestor and same segment.
DNA should support your genealogy, not the other way around. DNA is simply another record; a "data point" like a birthday or a residence.
Obviously adoptees have a different story. In everything we do, we are going from the known to the unknown. For adoptees, the only thing known might be their DNA matches. But once they establish some genealogy, the DNA should become supportive, not definitive.
This chart is solely for my own use as a reminder of the futility of chasing small segments. Can you really discern which of your 2048 ancestors gave you that 3CM segment? Do you have your tree documented all the way out 11 generations? No really, on EVERY one of those 2048 branches? Only when all three people in a triangulated match have all 2048 branches well documented out on every branch can you even consider where a 3cm segment *might* have come from. And even then, are you even reasonably sure?
So STOP chasing small segments.
(click for larger version)