Have to say that I try to get the ETAs within 3-4 minutes and often call with a correction if I've b@llsed up - but the latter is usually greeted with bemusement, so I guess they aren't that bothered. Undoubtedly more important when on London Info, though.
Can't see why people don't know to work out an ETA for the FIR boundary - that requirement is written in lots of places (including, I seem to remember, the guidance notes for writing a flight plan - don't you have to put ETE/LFFFxxxx in the remarks?)
Tim