If you are willing to apply for Economics (Hons.) at Bethune College, then you require at least a minimum of 88% marks. The index marks of Economics are calculated in a different way, so it is suggested to check the official college website for it.
If you are good at Mathematics, it is quite easy to get into colleges like Scottish Church College or Bethune College. If you have a mark of above 85% in Mathematics and English, it will be easier for you to get into a good college for B.Sc Economics Honours.