Remember that this noise will decrease as you increase the cell n's, and your power will also increase. Now let's look at our output. In this case, as usual in a repeated measures design, the Group x Time interaction is of primary interest. The relevant part of the initial output is shown below. Test Name Value Exact F Hypoth.
F statistics are exact.
If we are confident that our input values are correct for our target population, and if it is important to minimize n for this test, then we can fine-tune things. Of course, you usually don't know the means, common standard deviations, or correlation matrix. In that case, you need to start with a good guess based on pilot work or similar studies. After achieving the necessary power by the method above, it's important to run several analyses changing your guesses and noting how power changes as your guesses change.