A 99.7th percentile would imply a score between 2.7 and 2.8 standard deviations above the mean. IQ scores are standardized to have mean 100 and SD 15. So it would imply an IQ of between 140 and 142.
Technical: I assumed a normal distribution in the above conversion. True distributions of IQ scores are probably non-normal, but the tables I've seen for converting IQs to percentiles (include the one linked to above) always seem to be based on a normal distribution. If they converted an IQ score to a percentile assuming a normal distribution, then deconverting using the same assumption would make sense, even if the true distribution was non-normal.
