I think that you have to look at the application of your amp. Imagine that what-ever-there-is after your amp (your load) has a terrible CMRR and it can not accept cm variation larger than a certain value, for instance 100mV.
Imagine that your common mode is going to be set at 1V. The error coming from all sources cannot be larger than 100mV. You generate the 1V reference with a bandgap that is already introducing a 5% error (50mV). That means that your cmfb loop cannot introduce an error larger than 50mV too. So you need at least 26dB.