I think that was estimated based on what he had accomplished, I don't think he ever took an IQ test. Old IQ tests used a ratio IQ which was mental age divided by chronological age x100, but they stopped using that a long time ago, now they use a deviation IQ which compares your answers to individuals of your same age, and used that to calculate a score. That is why modern tests that go above 200 are so rare, they need a certain sample size of people in that range to develop the norms for calculating the score, and very few people have IQs that high. Also, I think there was a study that said deviation IQs which are more accurate than ratio IQs are usually much lower than ratio IQs, for example someone with a ratio IQ of 170 might have a deviation IQ of 140.