The "semi-legally defined" point at where the utility ends and the customer starts is typically at the service entrance {meter in most cases} so far as "voltage" {and voltage-drop calcs} are concerned. For 240V circuits and 230V utilization equipment, the agreed-on acceptable voltage is 208-254 {104-127} volts, preferring 220V {110} minimum if incandescent lighting is served. Some utilities tighten up on these numbers, but also say that it is a “best effort” goal and there may be conditions that do not allow that 100% or the time.
http://www.ari.org/std/individual/110-2002.pdf is an online standard that contains voltage ranges that utilities and utilization-equipment manufacturers agree on in most areas of the US. The pertinent material is §4 and table 1. [ANSI C84.1-1995 is the official standard, but it’s not freely downloadable like the ARI text.]
