Virtual feed to show total amps on dashboard

I created a virtual feed to calculate total amps, as follows:

Order Process Arg
1 Source Feed [ emonTx4_17: All usage]
2 / source feed [ emonTx4_17: Supply voltage]

I am able to view a graph of the output.
I created a dial in a dashboard using this virtual feed, however no data is shown.

What do I need to do to view the amps in a dial (ie realtime view of amp load)?