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Technical: I assumed a normal distribution in the above conversion. True distributions of IQ scores are probably non-normal,

The test is normed using their sample population. So functionally the scores are forced into the normal distribution. On the other hand, a population sample of 4800 (as used for WISC norm) starts to lose reliability when you get into the tails and the functional population at the SD+3 (IQ 145+) range is only 14 people total across the full testing age range.