Assuming an IQ of 100 and a window of 13.5:
You have to meet 2 people to find one inside of your window, and there is a 50.0% probability that this person will be more intelligent than you. When meeting a random person, there is a 20.2% chance that they will not understand you (below your lower limit), and there is a 20.2% chance that you will not understand them (above your upper limit). In a population of 1,000,000, there are 595,343 people inside of your window, 202,329 below your lower limit, and 202,329 above your upper limit.
How is this calculated?