I feel ya.. We have never had to deal with this before, and all I know is that my stepdaughter breaks down and cries every time we have the discussion. Hence, the attempts to go to a psychologist to help her deal with the issues...
Okay... enough... especially with the "I feel ya" crap.
I am a stepfather. A proud one. The kids my heart tells me to love are great. The heart does not make a distinction.
I certainly pay enough for both as well.
However, to one child, I have exactly the same legal rights as you do ... or my next door neighbor or that guy in the plane above.
None.
Even if.
You don't get to claim a title that doesn't belong to you and then get all pissy when the guy that DOES deserve the title wants it.
This is not a frigging playground. You are adding a level of complexity to this situation that is unforgivable.
You are not the father. You sit the girl down and say, "I love you, I will always love you and I will always be there for you. However, I am not your father... only the person that gave you half the genes that make you so special can be called that. It is the name he gets to be called because he actually participated in the miracle that created you. In the meantime, what do you want to call me... anything except smelly old guy has a fair chance... and all the other people that love me call me Don."
So will look at you, say, "So I can call you Don?" and that will be the end of that.
Stop staking a claim... you don't have a dog in this fight.
If you want to pull them out and measure them, at least do it where the kids won't be a party to it.