Every authority having power to issue a precept to a rating authority shall from time to time issue such precepts as will be sufficient to provide for such part of the total estimated expenditure to be incurred by the authority during the period in respect of which the precept is issued as is to be met out of moneys raised by rates, together with such additional amount as is in the opinion of the authority required to cover expenditure previously incurred, or to meet contingencies, or to defray any expenditure which may fall to be defrayed before the date on which the moneys to be received in respect of the next subsequent precept will become available.