One way is that you can select your fixed points by time. In the menu is an option for start time and end time. Set the start end end time, and then the statistics numbers at the top right of the window will then be calculated only for the time period that you specify.
I believe you must set your times to only include the best set of fix points in POSITION mode (next screen under (Gnd Trk). Then switch ALL to “1” for FIX only display. As I recall, this will then only display the FIX points and calculate the average of the “fixed” points only.
Yes, I meant to set start and end time in RTKPLOT, not RTKPOST. This way you post-process all the data and then analyze only the good portion of the solution.