I am basing my experiences of owning and driving a Fiat 500e rated by EPA at 84 miles. Usually 100% SOC can indicate upwards of 115-120 miles of range. Winter time closer of 90 miles. Of course, that's typically not what I'd actually get; all depends on where and how I drive the car: town vs highway, winter vs summer...
Here in SE Michigan, I have rarely seen the EPA rated 259 miles on the range display. It's usually less. In summer, it will get as high as ~240, and in winter, it can fall as low as ~160 when it's really cold.
It's all dependent on efficiency, as you know. The heating system can use a lot in winter (cabin heater up to 7 kW, battery heater another 2 kW), so short trips can result in worse efficiency / estimated range.
If you really want to measure battery degradation, you'll need a way to see the state of charge in 1% increments. Unfortunately, there is no built-in way to do this. You'll either need to use the MyChevrolet app with a subscription to see the Bolt's battery state of charge, or a bluetooth ODB2 reader with an app like Torque Pro (Android) or Engine Link or ODB Fusion (iOS). See
allev.info/boltpids for instructions and the list of PIDs to import.
To measure battery capacity:
1. Charge to 100%.
2. Drive until the battery state of charge is very low, below ~15%.
3. Note the energy used in kWh on the Energy screen of the Bolt.
4. Divide the energy used by the difference in the state of charge, e.g. 52 kWh / (100% - 13%) = 59.8 kWh.
Note that you'll get different results depending on the weather / ambient temperature. Winter will show a lower capacity than summer, so the most "accurate" time would be summer. For example, my largest recent drive used 37.4 kWh going from 100% to 38.4%, which translates to roughly 60.7 kWh. The Bolt has a nominal 64 kWh capacity, but my first drive test after getting the replacement battery showed ~62.7 kWh (I received a replacement battery during the recall in December 2022).
By that measure, my battery degradation over the past 2 years is roughly 3%, which is pretty good. As always, YMMV.