The sum charged on any estate under settlement in respect' of mansion and other buildings herein-before mentioned shall not exceed two years rental of the said estate, after deducting all public charges and interest of debts and other incumbrances and annuities affecting or which may affect the inheritance after the death of the limited owner, or, in the case of different estates settled to the same uses, and on which charges may have been imposed 'which affect the whole of such estates, after deducting from the rental of such of the said estates as may be charged with the cost of erecting mansion houses and appurtenances as aforesaid in the manner herein-after provided, so much of the debts and other incumbrances affecting the whole of the estates as shall bear to the whole of the said debts and incumbrances the. same proportion as the rental of the estates to be charged with the cost of erecting a mansion house and appurtenances shall bear to the rental of the whole of the estates settled to the same uses.