If you provide the sample size and the power that you would like your test to have, Minitab calculates the comparison rate you can detect with the test.

For the post office data, Minitab displays a power curve based on a sample size of 40. With a sample size of 40, the test can approximately detect comparison rates of 240 or 260 when power is 0.80. The test can approximately detect comparison rates of 239 or 261 when power is 0.90.