Enter the following: Vs, Vcm and R1 - R4. Then click on "Get Gains, Vo".
The Differential Signal Gain (Kdiff) should be R2/R1 if R4/R3=R2/R1.
Ideally, Vo due to the Common Mode Level should be 0V (Kcm=0).
But it's difficult to acheive with resistor tolerances.
Try setting R1=10100 (1% error), R3=10000, R2=R4=20000, vs=0 and vcm=5.
How much unwanted common-mode signal gets through to Vo?