This hack of an implementation for that indicator looks for the ScreenID: ETW_AggroPctN. However the value for a label showing the aggro percent comes from an EQType.
In the screenshot I moved the ScreenID to the layout box I use for the gauges:
Code:
<TileLayoutBox item="ETW_Gauge0InnerLayout">
<ScreenID>ETW_AggroPct0</ScreenID>
But the label itself uses the EQType:
Code:
<Label item="ETW_AggroPct0">
<EQType>314</EQType>
Note the box around the aggro percent label, that's a draw template to demonstrate what it's location and size is set to.