Properly sizing a frameset online (Ritchey Road Logic)?
I happen to be the exact same size, so I'll answer. I have a custom built road bike with 55cm c-t height and 57cm top tube. I'm using a 120mm horizontal stem and classic drop bars that have more reach than so-called ergo shape at the drops. If I for some reason wanted to have a stem with rise and ergo bars, I'd need even longer and lower frame. The 55cm Ritchey is a bit shorter and has a bit taller head tube but isn't that far off.
So, if you absolutely have to buy Ritchey, 55cm is probably the correct one unless know that your current frame fits perfectly and you want to change as little as possible.