In the event of the Gross Profit earned during any annual period of insurance (or during the accounting period of 12 months more nearly concurrent with any period of insurance) as certified by the Insured’s auditors being greater than the sum insured thereon, the Insured will be held covered to the extent of 10% of the sum insured hereof and a pro-rata additional premium not exceeding 10% of the premium paid on such sum insured for such period of insurance will be charged in respect of the difference.