Not an expert and not sure this will answer your query, however, just stating it

Please have a look at this site from money control. It has a field called contribution. When I add the contribution, I get the total of -276.54. This is exactly -0.73% as of Jan 21.

How exactly the contribution is calculated, I tried to search the net to find out but could not. Hope some experts will give a reply.

I tried to calculate Nifty 50 & Nifty MidCap Select Index % change, and calculated them almost accurately. Why only Bank Nifty calculations wont match up.

Can someone please tag a knowledgeable person here. Really not able to figure it out.