I had to download the AVM20 manual and poke around a bit to understand this. Anthem is unusual in their decision to basically disable (or at least override) distance/delay settings for dipole surrounds. I understand their reasoning (roof reflections are a very large variable in practical use with these speakers) and they are still doing it in the latest incarnations of the AVM platform, but I don't know of other companies doing this. I think I'd have to tinker with using this "dipole" mode and using my actual distances to see which I was more comfortable with, In your case, though, a difference of just one foot between the actual distance and the "dipole override" distance is so small that it's going to be almost impossible to hear any difference anyway.
if I actually enter a exact distance to the rear speakers, am I required to set a delay for those speakers becuase the preamp will not do it automatically?
The distance settings
are the delay settings - the processor applies delays based on the various speaker distances (roughly 1ms/ft) so that the signals all arrive at the same time.
In some articles that I have recently read the rule of thumb for setting delays suggest the fronts set to approx 25 while the rears / surrounds are set to approx 15.
The only "rules of thumb" I know are (1) to match the actual distances in your space and (2) to allow automatic calibration software to apply longer delays than measured for subwoofers if the subs are using any outboard EQ (such as the SMS-1) because those EQ's create a signal delay that processors' automatic speaker setup systems "see" as extra distance from the calibration microphone. Any rule of thumb that includes specific values is going to be fundamentally flawed, as the whole point of these adjustments is to integrate the system into our specific rooms.