How Estimate Capacity for a Battery If the Temperature is Changing Constantly
The formula assumes that the temperature is constant. If the battery will be subject to temperature changes that you cannot control, then the safest approach is to assume the worst case temperature (and the corresponding battery capacity) for the whole period. If you have some control and can predict a temperature profile, then you can use that profile to calculate an average battery capacity based on the temperature and the time at that temperature (this can be done piecewise if the profile is not too complicated but you could use an integral if you need more accuracy). By the way, do you know that the current draw of your device remains constant with temperature? If not, you should also take this into account.