I think that the CogAT uses a normal curve with a mean of 100 and a SD of 15, like most IQ scales. If that is correct, you can use http://www.measuringu.com/zcalcp.php to convert the percentile into a Z-score (choose one-sided for the proportion of area). Multiply the Z-score by 15 and add 100 and you should get the scaled score. So, for example, a percentile of 98% gives a Z-score of 2.05, which converts to a scaled score of 2.05*15+100=131.