The Frequency cursor of a Period window can be moved with high precision using the keyboard. First make sure you have selected the Period window of interest by clicking on it. If you now press the LeftArrow or RightArrow button, you will notice that the Frequency cursor moves over a very small distance to the left resp. right. Doing so, you can position the Frequency cursor exactly where you want it.

If you press the Shift button in combination with the LeftArrow or RightArrow button, the Frequency cursor will move over a slightly larger distance.