Each item insured under this memorandum is declared to be separately subject to the following condition of average :
If at the time of reinstatement the sum representing 80% (eighty percent) of the cost which would have been incurred in reinstatement if the whole of the property covered by such item had been destroyed exceeds the sum insured hereon at the breaking out of fire or at the commencement of any destruction of or damage to such property by any other peril hereby insured against then the insured shall be considered as being his own insurer for the difference between the sum insured representing the cost of reinstatement of the whole of the property and shall bear a rateable proportion of the loss accordingly.