Calculation of Special Alternate Minima. To calculate Special Alternate Minima:
(a) add 500 ft ceiling and 1.5 km visibility to the ceiling and visibility minima of the VHF runway aligned instrument approach procedure which provides the greatest advantage.
(b) add 400 ft ceiling and 1.5 km visibility to the ceiling and visibility minima of the VHF runway aligned instrument approach procedure to an alternative runway.
(c) when required by paragraph 8.1.10.3, add 300 ft ceiling and 1.5 km visibility to the ceiling and visibility minima of any other straight-in instrument approach procedure which utilises an aid other than those used in paragraphs 8.1.10.5(a) and 8.1.10.5(b) above.
8.1.10.6 The alternative minimum is the highest of those obtained above. Ceiling values should be rounded out to the nearest 50 ft. Visibility values should be rounded up to the next higher kilometre or half kilometre.
8.1.10.7 An extra margin over and above those mentioned above may be added if believed to be necessary because of peculiarities in the local weather patterns and/or difficulties in forecasting.
Might be easier to use the standard alternate minima.