If you provide a power value and the sample size, Minitab calculates the value of the comparison proportion. If you provide values for power value and comparison proportion, Minitab calculates the value of the sample size.
For these two cases, Minitab uses an iterative algorithm with the power equation. At each iteration, Minitab evaluates the power for a trial sample size or trial proportion value, and stops when it reaches the values you specify.
When Minitab calculates sample size, it may find that no integer value of sample size yields your target power. In such cases, Minitab displays the target value for power alongside the actual power, which is a value corresponding to an integer sample size, and which is nearest to, yet greater than, the target value.