1. Every year the regulator shall compare the average activity level of each sub-installation, determined in accordance with Article 4, with the historical activity level initially used to determine the free allocation. When the absolute value of the difference between the average activity level and the historical activity level of that sub-installation is more than 15 %, the free allocation of allowances in respect of the sub-installation shall be adjusted. That adjustment shall apply as of the year following the two calendars years used for determining the average activity level, and provided that the adjustment of the annual preliminary amount of emission allowances allocated for free to the sub-installation is at least 100 emission allowances. The regulator must calculate that adjustment by increasing or decreasing the free allocation for the relevant sub-installation by the exact percentage change in the average activity level compared to the historical activity level initially used to determine the free allocation.