This should work, however you will need to stop emonhub first sudo service emonhub stop
However, there is currently an issue where your calibration will be overwritten each time the emonPi / Emoncms is updated. We are currently working on a solution for this see:
In the meantime a better (and easier IMO) way to calibrate the emonPi is to edit emonhub.conf. This can be done via local Emoncms Setup > EmonHub > Edit .
To enter the scalor for the emonPi you will need to edit the relevant value node ID 5 e.g
Another way to apply calibration is via Emoncms Input Processing