The dilution of Elven blood and subsequent mortality

[Belegorn]

Originally Posted by The Mouth of Sauron

Imrahil of Dol Amroth "had Elven blood in his veins" but was mortal. Does this mean that as the proportion of such blood is diminished in succeeding generations via marriage with mortals, that the bloodline reaches a point where death happens? And at what stage does that happen? 75% 50% 25% ? And why aren't some of Imrahil's ancestors still alive, if they had a high proportion of Elven blood?

So far as I know only Elrond's line, including his parents and brother were allowed to choose a Doom. Anyone else is mortal. Imrahil's ancestors were Númenóreans/Dúnedain. The King [Aldarion; 6th King] of Númenor's wife, Erendis, called the Men of Númenor half-Elves, especially the Kings and their relatives, but they were Men.