There's no hard and fast rule that converts an index number to a start group. They will determine the cut off index values for each start group based on the field that has entered and then split the indexes into appropriate/desired group sizes. So if last year's field had more riders with higher indexes, the index cut off for each group will be closer, if that makes any sense. An extreme example, if there are 500 riders with indexes below 10, but they want to limit the bunch size to 100, the cuttoff for the first group will probably be 1 or 2. Compared to if the same 500 riders have indexes spread across 0 to 20, the cutoff for the first group will prob be around 3 or 4. So even if your index doesn't change, you can either drop or go up a group.
