I have problems in the cutoff frequency and gain values (LTspice)

I have a problem in the cutoff frequency values and the gain I must have 
in the gain G=51 and f=51Hz if you have noticed me, I have values that 
are far from that it is a circuit of an EEG and this part of the circuit 
is a common mode rejection and I have another question about AC which is 
at the entrance does it always have to be AC=1 or something here I took 
AC=20 because when I took AC=1 I got found in the gain G=0

someone can help me?