When the accumulation of the sinking fund for any year shall be more than three and one-half per cent, then the amount in excess of three and one-half per cent may be applied to reduce the sinking fund requirements for the succeeding year, and if the accumulations of the sinking fund for any year are less than three and one-half per cent, then an amount sufficient to make up the deficit in accumulations for that year shall be added to the sinking fund requirements for the succeeding year.