One - to - One matches
It is useful to know that experts consider all matches, however short, "matches" but if you go below 3cM you have almost no chance of finding out what the family tree connection will be. Generally under 7cM you are going to find it really difficult too. In these cases you need to have lots of other evidence, using your tree and theirs, surnames, geography and timelines in common too. Possibly if you have several people with a small size match you have several trees to check against. I personally do not use matches below 6-7cM or say 0.3% to contact others, unless there is a very good other reason.
Most websites provide a generation estimate - how many generations back is your common ancestor. Each provider is different but you should be able to get a generation result, percentage match of DNA, how many pieces of DNA match (segments) and a family tree.
One way to understand the generation estimate is to count the steps it takes to move from you to a known relative on the chart, like stepping stones?
Percentage DNA easily becomes small when moving across the chart from your position. The general 'autosomal' DNA test is useful for a maximum of 6-8 generations in perfect conditions. Something not always explained to potential purchasers of DNA kits.
Remember that a DNA match helps with trees but it is a two way street, using the tree and the DNA together? It is possible to match trees and then look for DNA confirmation. Wikitree allows users to put their kit details alongisde their trees. Finding a useful tree then allows a DNA comparison to be made. On-site there is a facility to identify living descendants who could be available for testing.
Generally speaking, only contact your matches with a tree and some extra information that is relevant to you. I made a speculative leap with a small DNA match, no reason to make contact except her surnames list was 90% of mine. Turns out her ancestors lived within 1-2 km (about a mile) from mine. However, that is the exception rather than the rule and we still have not found a direct connection. :)
MyHeritage suggests using anyone over 2% (2nd or 3rd cousins or within 4-5 generational steps) - once you have tested and compared close relatives. These are the ones who you are most likely to find a common ancestor from your unknowns. There are no given strategies from this point. Each of you will have different information of different types and it relies on a lot of conversation and mutual cooperation between you. Trees, Geography & Timelines are all important cornerstones but any information, even family stories and family traditions, particular spoken expressions are all important too. Write everything down however small. It is surprising how useful these notes are later on. Bear in mind that many people will not return your email? There are many reasons but your most interesting leads can be frustratingly, not interested. 'C'est la vie' of the genealogist life!
This description becomes more important with the reducing size of matches (below 7cM) and it becomes more important to prioritise which match to connect with. You need to consider the lack of potential success combined with the amount of effort needed to find a match against how important that match is to you?.
Seamus Crowe Website2021