It really depends on what your comfortability level is and how much electricity you're willing to use. Experiment.

We turned ours on at 30 F outside.

As long as your heat pump is working properly, you should not need to turn on emergency heat. The heat pump will use the heat coils when it is unable to maintain the set temperature, as when it is too cold outside. Emergency heat relies totally on the heat coils and is normally used only when the heat pump malfunctions.

most women, in my experience, hit the emergency @ about 35 outside.

The old answer differs somewhat from the new one. In the past before the latest technology, we used to install a "OT" outdoor thermostat and it mechanically changed your heat pump system from heat pump operation to electric strip heater operation consuming muck more electricity to heat your home and it was adjustable. Now days we do not use the outdoor temperature to decide when to bring on our electric strip heaters. We now use electronic thermostats that do all of this for us. I am a fan of the Honeywell Focus Pro 5000 (6000 for programmable options) for all of my installations because it atomically turns on the back up heat weather it is electric strip heaters or gas or even fuel oil. And best of all the Focus Pro sells for about $80. Operation- This is as common as I can make it, when your new thermostat calls for heat, the FocusPro will allow the heat pump to come on and operate. If the thermostat does not see a temperature rise (as low as .02 degrees) over a period of 30 minutes, it will bring on the electric strip heaters to satisfy the call for heat. Now here is the neat part. The next call for heat, the heat pump begins the operation but the thermostat will only the heat pump to run approx 28 minutes to satisfy the call for heat before it allows the strip heaters to come on. Guess what, the next time the thermostat calls for heat it will only allow the heat pump to run a shorter amount of time to satisfy the call for heat. Where I am going to with this is the thermostat is “learning” what the outdoor temp is by how long it takes to heat your home (satisfy the call for heat). On a 0 degree day, Your thermostat may only let the heat pump run for 1 minute before switching over to your backup heat. But the first time that your heat pump achieves the call for heat, the FocusPro will allow the heat Pump to run a longer period of time to achieve the call for heat. I am only a service technician for a HVAC company and I a sure there is a lot of other things that the FocusPro takes into consideration to make these changes, but from my experience that is the short version of how the thermostat works. I have noticed that most electronic thermostat do this the same way, eliminating the outdoor thermostat.