R-DF27 is a prolific lineage of R1b that formed 2500 BC.
According to Sole-Morata, Villaescusa et alii, the origin is NE Iberia. The high percentage of DF27 within Basques (supposedly up to 70%) is explained as a founder effect due to the lower diversity of DF27 found there.
The Wikipedia article says the origin is NW Iberia - "It is estimated to have developed around 4,200 years ago in north-western Prehistoric Iberia as the Neolithic made way for the Atlantic Bronze Age."
PhyloGeographer arrived at Calais by taking the midpoint of P312's initial average location (computed recursively through its subclades) and the centroid formed by the directionality-refined computed origins of thirty-nine DF27 child clades along with five basal samples.
Keep in mind that a completely data-driven approach like this is not guaranteed to produce a result that is correct.
The algorithm is completely agnostic of coastlines and trends in human history. The inputs to the algorithm consist of a tree with formation and TMRCA dates at each branch and a set of geolocated, dated samples.
Some complicating factors are at play, including higher than average testing in the British Isles and the US, lack of regional specificity of many YFull samples, and the complexity of myriad, crisscrossing subsequent migrations in the 2500 years since DF27 formed.
A back of the envelope count I made of how many of the DF27 lineages have English Channel / Scandinavian / Canadian / US presence vs Iberian / Latin American / Pyrenees France was inconclusive at 30-30.
Given the aforementioned complications, I am unsure of the reliability of the computed result and am not advocating any specific migration theory.
However, something I find compelling about this result is that an algorithm that is agnostic of coastlines ends up at the closest spot to England on the continent.
I'm not an expert on the archaeology of Bell Beakers who supposedly migrated to the British Isles at this time and are associated with some haplogroups of R1b. It is conceivable to me that, whether Bell Beaker or not, some of the DF27 may have migrated to England from Pas de Calais.
In spite of the lack of certainty, I believe that we are starting to have enough data for some haplogroups to power data-driven calculations like PhyloGeographer. Let's keep watching DF27 and see how it evolves.