Section 24. Every such corporation shall set aside at the end of each calendar year out of its surplus funds, if any, but not otherwise, a special contingent reserve fund in an amount equal to, for the first year, the sum of one half of its then surplus funds, over and above its liabilities, and three per cent of its net premium written during the current calendar year, which three per cent of premium written, however, shall not exceed fifteen per cent of its incurred losses during such calendar year; thereafter, it shall at the end of each succeeding calendar year from any available surplus funds so increase such contingent reserve fund that on December thirty-first of each such year the amount of the contingent reserve fund shall exceed the required amount on the next preceding December thirty-first by an amount equal to five per cent of the net premium income of such corporation during such whole calendar year; provided, however, that the contingent reserve fund at the end of any calendar year shall not exceed forty per cent of its incurred losses during such calendar year. Such contingent reserve fund may be withdrawn or reduced below the amount required by this section with the approval of the commissioner of insurance. No other reserves except for actual liabilities shall be required.