I understand that balancing is a factor but I don't believe the system is too far out because when the AC is having to work, during the day, the temperature is fairly even. At night, unless it's very warm outside, the AC tends to stop cycling and this is when it ends up far warmer upstairs.
Actually the fan does a good job correcting the temperature difference but I don't wish to have it running 24/7.
Also, there are substantial periods of time throughout the year when neither AC or heat are required. It would be nice to be able to automate fan operation with the goal of equalising temperature. I realise you can get thermostats that will cycle the fan but without temperature differential feedback run time cannot be optimised.
Why don't you want to run the fan in continuous mode? It is much better for your home overall to do this. Filters are more effective, hot and cold spots are less likely and fresh air (if equipped) is constantly being cycled. For the times of the year when neither the A/C or heat is needed, but you want to run the fan, temperature feedback is not needed; the fan just runs.
Using something like the C-750 (
CC-750 comfort control center) to sloww down the RPM to 400-600 RPM is a great way to keep continuous fan operation while using less energy to do it.
If you have a system with an ECM motor (or X-13) you can use the Honeywell IAQ stat to control RPM in the same way (
Honeywell Vision Pro, IAQ). this model will accept a remote indoor sensor, but will not average the sensors. If you have multiple sensor locations (min of three required) it is possible to wire them in series parallel (they are just resistors after all) to achieve the same thing as true averaging. For true averaging you will need to step up to a commercial grade stat.
Averaging is not going to be the best option though; however, if you only need it at night (and you sleep upstairs) it will probably work ok for comfort although you may see higher utility bills.
The best option is to find a good HVAC contractor who is WELL versed in multi-stage equipment zoning. You will see both better comfort and lower utility bills. Depending on your location I may be able to make recommendations.