ECG: gain of right leg drive?
According to this (page 5), the common mode rejection ratio is improved by a factor of \$1 + A\$ of the closed loop gain, where \$A = Z_f / R_cm\$.

You have buffered the \$R_cm\$ out of being an input to your system, so your's is going to be \$R6\$ instead.

It seems that there is no "best gain".  A larger gain improves the common mode rejection ratio, giving better signal, however as you improve the gain, you might run into stability problems with the cable impedance, and you will need to compensate for that, and you will decrease the bandwidth of your op-amp, as you increase the gain.