The four sails are fixed to two spars in pairs. The inner spar, with a pair of sails, is closest to the mill tower with the outer spar with its pair of sails just in front.The optimum windmill speed (sail speed) is between 12.5 and 15 revolutions per minute The Miller however does not count complete revolutions of the sails. Counting is done by sails (or ends) passing. Four ends make up one revolution. The best milling speed for Mostert's Mill is between 50 and 60 ends per minute.
With varying wind speeds the Miller has to continually check the speed of the sails and make adjustments. From experience the Miller will gauge how much sailcloth is initially required when starting to mill. The sailcloths (canvas) are always spread in pairs (so that the opposite sails carry the same amount of sail).
When strong winds cause excessive speed, the Miller will "reef" the sails (reduce the area of canvas) to maintain the optimum milling speed. There are five stages of sail setting (illustrated below) and combinations of the five can be used to optimise the speed. The same sail setting will always be used on a pair of sails but the other pair could have a different setting.