Besides keeping the interior temperature at a certain level, for example 22 °C, the car's heater is also used to bring the temperature at that level in the first place. This happens when no preheating was applied. For example, when the car is at temperature +4 °C when you leave, then the heater has to increase the temperature by 18 °C until it is 22 °C. The amount of energy needed for this depends proportionally on the difference in temperature with a rate that is called the heat capacity of the car. The heat capacity is a parameter that indicates the amount of energy needed to increase the car's interior temperature by 1 °C.
Note the difference with the loss rate in the above post. Loss rate depends on the surface or 'skin' of the car's interior, such as the roof, the windows, the floor, the doors, and how well insulated this skin is. In contrast, capacity depends not on the surface, but on the volume and/or mass of the interior, how much heat energy the interior in a sense stores or absorbs by heating it. If some material stores a lot of energy, you need a lot of heat to let the temperature rise, if it stores less energy, less heat is needed to let it increase.
When for some cases you know the amount of heat delivered by the heat pump and/or resistance heater, and also how many degrees °C the interior has increased in temperature, you can estimate the heat capacity parameter. For example, when for 5 minutes the heater works at 6 kW, then the total amount of produced energy can be calculated by 5*6/60 = 0.5 kWh. This energy is taken from the battery, so that for a two hour trip as in the above post your battery spends a total of 1.0 (due to maintaining) + 0.5 (due to initial increase)= 1.5 kWh in total on heating during the whole trip. This is 1.5/28 = 0.054 of the whole battery, which is 5.4 %. By this percentage also your range will be reduced.
Assuming the increase in temperature was 18 °C (from 4 to 22), and the heat was produced only by the resistance heater, this gives an estimate for the heat capacity parameter of 0.5/18 = 0.028 kWh per °C increase. However, when the car also has a heat pump, then this will probably also contribute to the increase. As it increases very fast in only a few minutes time, it is probably not only the heat pump; maybe most (or almost the whole) of the increase is due to the resistance heater.