You haven't quoted the full paragraph which is:
(b) When showing compliance with subparagraph (a) above, an operator must take account of the following:
(1) The altitude at the aerodrome;
(2) Not more than 50% of the head-wind component or not less than 150% of the tailwind component; and
(3) The runway slope in the direction of landing if greater than +/-2%.
(c) When showing compliance with subparagraph (a) above, it must be assumed that:
(1) The aeroplane will land on the most favourable runway, in still air; and
(2) The aeroplane will land on the runway most likely to be assigned considering the probable wind speed and direction and the ground handling characteristics of the aeroplane, and considering other conditions such as landing aids and terrain. (See IEM OPS 1.515(c).)
Which means that, when considering your performance planning before flight, you have to consider the most limiting of both the still air and forecast wind conditions and, when you consider forecast wind, you must factor it to use only half the headwind and one and a half times the tailwind.