Thanks everyone for your help.
Nobody's queried my figure of just over 8.5% a year compound growth, so it's probably not far out.
It's occurred to me that I took it for granted Woody reinvested all his customers' dividends, But when it comes to investment you can't assume a goddamn thing.
Did he reinvest them?
If he really achieved that rate of growth while trousering the dividends, even I'd be forced to agree he must be a bit of a genius.