Your CMRR equation is wrong. CMRR is not the same as signal to noise -- and has little to do with it, in fact (though poor CMRR can result in noise).
When building a differential amplifier, like EEG, there are two gain "modes" (not a formally correct term) -- there is differential gain, Ad, which you want, and there is common mode gain (Ac), which you don't.
CMRR is 20log10(Ad/Ac), and you want this to be a big number. To get the common mode gain, you put the SAME SIGNAL into both inputs of your amplifier. Say you put a 100 mVolt sine wave in to both inputs. Ideally, Ac is zero, and CMRR is infinite. Lets say, though, that you get 1 mVolt sine wave out. Now, Ac is 0.01. For a differential gain of 10000, CMRR is then 20log10(10000/0.01), and you have 120dB CMRR. Before you ask, yes, increasing the Ad usually increases CMRR, almost by definition.
The ONLY way to measure CMRR is to put the same non-zero signal into both differential inputs of your amplifier, and observe the output.