In case you reconsider using the driver curve – where you can also do this kind of complex mapping – I explain it in here:
I’m on my phone so I can’t test it, but I think a boolean-style property like bone visibility should get a value that’s either 0.0 or 1.0. That’s what I’d try first.
Edit: yeah 0.0 and 1.0 seems right, @JA12 said that in a post way above.