I wonder what the power company uses to measure line voltage.
Doug,
I bought a Simpson 260, the first piece of electronic gear I ever owned, for a graduation present to myself in 1950. The a-c voltage accuracy was 3% then, probably closer to 5% today, given its copper oxide rectifier and component ageing. At half-scale on the ridiculous Simpson 25 - 100 - 250 range selections the typical (which means nothing) accuracy could be as bad as 10%.
Your accurate peak reading converted to RMS is probably the best you can do, and should be pretty close, given waveform purity presently maintained. The UPS measurement is suspect because of waveform distortion and lack of knowledege about the metering circuit.
The power company probably has a $5000 certified expanded scale chart recorder "somewhere" in the system, but then you don't know where you are on the power line or what its regulation is at any time of the day.
Next big hamfest pick up a Weston mirror-scale meter with an 8" scale in a nice mahogony box for $25 tops. They knew something about measurement.
Bob - NE