The department must give notice to an employing unit that has failed to make reports required pursuant to Chapters 27 through 41 of this title, or has filed incorrect or insufficient reports. If the employing unit refuses or neglects to file a proper report within fifteen days after notice has been mailed to it, the department shall determine the amount of the wages payable for employment by the employing unit for the period for which the report is required. The determination must be based upon the department’s best information and belief. The department must then determine the amount of contribution due, if any, computing it at double the rate which would otherwise apply. The department may allow further time, not to exceed an additional fifteen days, for filing the proper report after notice is mailed.