You don't need a 50% blood line to maintain your nobility title, you inherit the title from someone else, usually your father just like, and usually along with, a house or an estate.
To make it like aristocracy the rule should apply recursively, you are a Mohawk if 50% of your ascendants are Mohawks as defined by the same rule.
I once new a guy who was working really hard to keep a blood % out of his tribe treaty in Washington state. Said it was really hard to get his tribe to realize just how valuable it was to only have to show lineage, and not how much. The US (state?) government wanted to pay a small sum of money as due consideration for the treaty change, and he just couldn’t get the tribe to understand that the money would be gone in less than a few years, but that their grandchildren would likely have a harder time. I don’t think it helped that this guy looked pretty white, and wonder if there were people in the tribe who thought this % clause would help keep people out of the tribe who maybe didn’t look like they belonged.
To make it like aristocracy the rule should apply recursively, you are a Mohawk if 50% of your ascendants are Mohawks as defined by the same rule.