All such property shall be assessed and taxed as follows:
If the right to exemption ceased in any tax year, the property shall be assessed and taxed as of the first day of the month following the date when the right to exemption ceased, for the proportionate part of the said year then remaining. The amount of tax shall be determined by multiplying the amount which the tax would be if such tax were for the entire year by the number of whole months remaining in the calendar year after the date when the right to exemption ceased and dividing the result by 12.