David,
The bbConfigureProcUnits() refers to the video processing units, “Video Units” in the Spike software. For channel power you will want to set this value to power as you were. The detector should be set to average, and the scale in the bbConfigureAcquisition() function should be set to BB_LOG_SCALE. You have the scale set to BB_LIN_SCALE, which is what changes the output units from dBm to mV.
Regards