Not sure what you mean by 'round robin'.
All ai flight plans are 'round robin' and, regardless of how many scheduled stops there are along the way, must be scheduled to return to the originating airport within the specified 'repeat period'.
As for 'flight times'. Unless you overide the arrival time (@ prefix in TTools) the compiler calculates the 'arrival time' based on distance and the cruise speed entered in the 'aircraft.txt' file. The 'calculated' arrival time may be different than what you entered. Also arrival times are generally 'in the airspace', and depending on weather and traffic, landing time can take 15 minutes or more.
What sim and what program are you using (TTools, AIFP, ??)
At that time  the chief engineer was almost always the chief test pilot as well. That had the fortunate result of eliminating poor engineering early in aviation.