I'd assumed a perfect sinewave, but of course that is more often than not the case. I've got this distant memory of the RMS being .7 of the peak. Heck, for telly mending, we didn't have to get things more than vaguely right.
So, 240v mains X 1.42857 would be 342.857 - ish. That seems high. Perhaps it was .77, which gives an answer closer to my memory. The1950s seem a long way away sometimes.
(Not that our mains is 240 anymore.)