Calculate the differential common-mode gain of instrumentation amplifier
For ideal matched parts with shared Rgain there is (in theory) no CM gain.
\$V_{out} = (V_2  – V_1 ) (1 + \dfrac{2R_1}{R_g})\cdot \dfrac{R_3}{R_2}\$
The method of computing sensitivity of any result is a partial derivative with respect to the variable.
In this case the rise in CMRR is due to mismatch tolerance worst case.
The partial derivative of output w.r.t. R1 \$= \dfrac{2R_3}{R_2}\$
The tolerance on a single R1 of 0.2% with R3/R2=1 is 1/1000 or CMRR=60dB
For +/-0.2% on each R1 CMRR reduces by 6dB or CMRR=54dB