If a thermostat has aux heat lock out, or if using an outdoor stat to lock out the aux heat. A balance point graph that uses the load calc and the heat pumps performance specs.
On homes where the owner just wants to upgrade to a stat that can lock out aux heat. A best guess based on other homes in the area. Along with the instructions to the HO on how to reset the lock out temp.
Keep in mind, that a home that has the aux heat locked out until 20°F, may do fine during the day, but lose a few degrees temp during the night, due to loss of solar heat gain.
Aux lock out benefits those people that like it a bit cooler when they sleep, then during the day. And don't want to constantly be paying for the aux heat to do the heat recovery work, that the heat pump can do, but takes much longer to do.