On a Mercator projection the meridians are parallel. This means that, regardless of latitude, all parallels of latitude will be the same length; in this case 133cm.

Scale at 30ºS = Chart distance / Earth Distance = 133cm / 360x60xcos30ºnm
= 133cm / 18,706nm = 133cm / 34,673.8km = 1/26,070,521
nearest answer is 1/25,000,000.