Need help finding an unknown coordinate, for an x-axis value

[Spaghetti scientist]

Hey all, I'm new to this forum thing, hopefully someone can help
I have created a curved scatter graph almost sigmoidal, I am trying to find the midpoint coordinate so I can read off an unknown x-value.
I have a known Y-value, but do not even know where to begin. I am trying to find the Kd of a protein ligand interaction.
Can anyone help me please ?

[AliGW]

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