You should not depend on the range guesstimate which can change drastically based on many factors, including firmware updates which could (have?) changed the guesstimate algorithms. Instead, you should track distance driven per kWh consumed under identical driving conditions (not easy). Or maybe the total battery capacity as reported in the (secret) service menu is reliable enough to determine whether battery pack capacity has been lost (anyone know?).
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE