While rooting around in the list archives, I happened on a 2006 post from the late, great Stan Kelly-Bootle (we miss you, Stan!) where he checked VN's math from the following passage in SM, and found it correct:
"A foolish tutor had explained logarithms to me much too early, and I had read (in a British publication, the Boy's Own Paper, I believe) about a certain Hindu calculator who in exactly two seconds could find the seventeenth root of, say, 3529471145760275132301897342055866171392 (I am not sure if I have got this right; anyway the root was 212)"
I did a little searching and was able to find VN's probable source, which was not (or likely not) the Boy's Own Paper, but actually a much later publication, the 1931 Proceedings of the Society for Psychical Research. In Google Books I was only able to access the "snippet view," so I don't have the whole passage, but I was able to see the following:
A HINDU ARITHMETICAL GENIUS In the American spiritualistic journal 'Immortality' for June-July 1931 appears a short account ... If the facts are as stated, it may be admitted that he has a claim to be regarded as the greatest mathematical prodigy the world has ever seen. ... He found the 17th root of the number 3529471145760275132301897342055866171392 in one second, giving it as 212.
VN was an enthusiastic reader of the SPR's proceedings, so it's not hard to imagine him writing down this anecdote for later use.