I say probabilities should govern this, not some set value. If an Elf and a Human hook up, then a Half-Elf should just be one possibility. Something like 50% Half-Elf, 25% Human, 25% Elf. Then, in turn, the hybrids should "lock in" to their races, meaning a Half-Elf can only mate with an Elf or a Human. Their child, in turn, would be 50% hybrid, 50% Elf or Human (depending on partner).
I'm just making up numbers as I go along, but the point is, in order to avoid hybrids from taking over, there should be some systems in place. And like the "real world", hybrids should be discriminated against by some people/groups...perhaps even wars could start over it.