OK, the rule of thumb is:
1.1 V rms for an emonTx or anything running off 3.3 V
1.6 V rms for an Arduino or anything running off 5 V.
Those numbers take into account tolerances on the transformers (voltage or current), the bias chain offset and voltage divider or burden resistor tolerances, plus a few percent for waveform distortion. The reasons for not allowing 3 times are (1) the voltage wave won’t be anywhere near that in its crest factor, and a high current load that approaches the peak design current is much more likely* to have a wave shape that is close to the shape of the voltage wave, and (2) most people require good accuracy at low currents, so throwing away a third (or whatever) of the resolution is for most people a bad idea.
If you are designing your system properly, and you know the current wave shape, then the right way to do it is to use the peak value that you know about, and add the circuit tolerances on top of that.
*[The high current load is likely to be well-behaved because of legislation regarding harmonic pollution of the supply.]