6For the purpose of determining the loss on account of a parish rate, the foregoing rules shall have effect as if the expression " rates " meant such parish rate, and the expression " separately rated area " meant the parish or the part of a parish in which such parish rate is levied; and the loss on account of parish rates of a district shall be the aggregate of the losses on account of parish rates of the parishes or parts of parishes in which parish rates are levied within the district.