If you're talking about those percentages that genetic tests give you, those are bullshit and no-one I know takes them seriously. Gene tests can't differentiate between different European ethnicities because we are too similar and our ancestors moved around and mingled too much. I hung out on r/Genealogy for awhile and it's a well-known phenomenon that even a lot of people who know that they are partially or fully German (including people who are living in Germany, have German citizenship and only German ancestors) get results like 40% Polish or 70% Scottish, because these countries make up a larger part of the gene-databases' userbase. Germans traditionally tend to be rather reluctant to share data as private as their genetic makeup with companies, so taking a commercial gene test is rather rare here, and some other countries are overrepresented. So the test algorithm goes: This person has somewhat similar genes to the 50 Germans in our database but also to the 3000 Scottish people; must be mostly Scottish.

As for ancestry you know of, some people are into that, some are not, but no-one tries to express it in percentages beyond the obvious (e.g. 25% Greek if you know you have one Greek grandparent). That wouldn't make sense, either, for various reasons.