How to convert a pi attenuator to a tee attenuator?

I need to convert a pi attenuator into a tee attenuator. I found this online http://en.wikipedia.org/wiki/File:Pi_Pad_converstion_to_or_from_Tee_Pad.jpg#filelinks But it seems wrong to me seeing as to calculate Rc, you need Rc. This is the only resource i can find online, which is annoying because it's wikipedia lol.

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