They showed a lower bound of .1 points, not a total .1 point decline; the most reasonable extrapolation would be .3 points, so since the 1910 start of their sam

They showed a lower bound of .1 points, not a total .1 point decline; the most reasonable extrapolation would be .3 points, so since the 1910 start of their sample, -3.21 points (and possibly more since the phenotypic correlations driving this decline were noted for decades before then in England, but at the moment, while we have ancient genomes from thousands of years ago and many modern genomes from 1910AD on from 80yos+, 1000-1900AD is a bit of a doughnut hole - not so ancient people are willing to sequence without 'consent', but too ancient to ever get consent).