Worth reminding observers that if there is gap between the NLC and the horizon that it's a good idea to record the height - the 'h' angle, from horizon to base of NLC. Having said that, I've seldom seen this; more often the NLC sinks into low murk or is present right down to the horizon.
Similarly, I've seldom seen a 'base' as such. My own measurements are simply of the maximum upper limit during the display period, which seems to be a good indicator of meso temperature. Brightness and complexity of the display are too subjective to be of any real use.