Section 11. When the amount or value of the subject-matter of any bond, mortgage-deed or settlement chargeable u

11. When the amount or value of the subject-matter of any bond, mortgage-deed or settlement chargeable under this Act with an *ad valorem* stamp-duty and referred to or mentioned in section six cannot be ascertained, the proper stamp to be borne by such instrument may be determined by the person bound under that section to bear the expense of providing the stamp: Provided that, under such instrument, nothing shall be recoverable more than the highest amount or value for which, if stated in an instrument of the same denomination, the stamp actually used under such option would have been sufficient.