Hi Dave,
There's some discussion of the Pickering seeing scale here:
www.telescope-optics.net/induced.htm
but I find Damian Peach's animations most useful:
www.damianpeach.com/pickering.htm
NELM is just "Naked Eye Limiting Magnitude". I should measure it at the zenith, but I cheat and always use Ursa Minor, so my readings are somewhat pessimistic in winter (when Ursa Minor hangs down from Polaris).
The SQM reading is a less subjective measurement of transparency. Technically the readings are in mag/square-arc-second, but pretty much everyone uses the "Sky Quality Meter" from Unihedron to measure it (thus the somewhat colloquial "SQM" label).
unihedron.com/projects/darksky/
Mattias Wirth/APM built my Mak-Cass; it uses Intes Micro optics (in AstroSital).
Cheers,
Jeff.
