1st day of the group:
summation of all ratios / # of stocks in group * groups prior close (should be 100).

After 1st day:
summation of all ratios / # stocks in group * groups prior close

High, low and open: (example is for high)
current high / current close = ratio
summation of all ratios / # of stocks in group * current close for the group

Note: Sectors are computed using the same method, except the close refers to the close of the groups within the sector.