The amount of the bond filed by the customs warehouse proprietor, as required by §44.243, shall be not less than the estimated amount of tax which may at any time constitute a charge against the bond: Provided, That the amount of any such bond (or the total amount where original and strengthening bonds are filed) shall not exceed $25,000 nor be less than $1,000. The charges against such bond shall be subject to increase as withdrawals are made and decrease as required evidence of exportation is received by the appropriate TTB officer with respect to cigars withdrawn. When the limit of liability under a bond given in less than the maximum amount has been reached, further withdrawals shall not be made thereunder until a strengthening or superseding bond is filed as required by §44.245 or §44.246.