Following math will help you make unbiased predictions, which will therefore be more accurate.
Start with an estimate of the prediction you need to make.
This estimate should typically be around the average value of the subject you are predicting. For example, if you are trying to predict someone’s GPA, start with an average GPA score.
Determine the prediction that matches your impression of the evidence.
For example, if you have evidence that a girl could read fluently by the age of 4, you may increase your GPA prediction for her, as her high reading level probably means a high level of intelligence.
Estimate the correlation between your evidence and the subject of prediction.
This correlation can be obtained from the number of shared factors. Shared factors are any factors that are shared by both your evidence and your subject of prediction—for example, any factors that cause children who are good readers to become academically successful. One shared factor is genetically determined aptitude.
In this case, the correlation between a high reading level and GPA scores would be about 30%.
Move the correlation value of the distance from your original estimate of the prediction to the adjusted estimate of the prediction.
In this case, move 30% of the distance from the original estimate of the GPA to the estimate of the GPA after adjusting for the girl’s reading level.