Appears the Dude cannot handle negative values when creating Functions (returns a HUGE number).
Very similar to what was mentioned long ago in this thread (an issue that was apparently fixed)…
http://forum.mikrotik.com/t/converting-string-into-int/10193/1
I realize multiplying the value by -1 gets around this for RSSI values.
But doesn’t help me with respect to cable modem receive values that hover around the zero mark (not all modems have negative rx values).
Hope this can be fixed in future version.
Thanks for a great product 
Dave