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I think its not so much a matter of percentage of admixture per se, but whether or not a particular phenotypical trait will dominate for a certain period of time over another. The longer the phenotypical trait is left in the area among other traits that are unlike it, the greater chance it will be bred out within a certain period of time.
That's why early Italics looked standard Dinaro Med. because the IE invasions happened at 3400 BC in N. Italy. The more 'Kurganid' traits associated with PIE speakers (that is working within the confines of the Kurgan culture for IE invasion of Italy) would have thus been reduced since the process was one of elite dominance more and more local autosomal traits dominating throughout the pennisula but also cause it was so far back in time.
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