[Arran]

With regard to voltage readings sometimes the schematic will tell you what percentage of possible error to expect in that they could not account for differences in line voltage and in volt meter calibration. The older ones would often tell you how many ohms per volt the meter used to compile the readings would have loaded the voltage down.
However it's been my observation that most voltage charts from the mid 1930s onward appear to have been compiled using a VTVM as the readings seem to be pretty close to what my DMM reads. A VTVM is something which any good factory or electrical engineering lab would have had at that time, in such cases the volt meter would not load down the voltage reading much if at all.
Regards
Arran