This is from an old post, I think it's in the game docs somewhere.
The index can fluctuate between 90 (and in some cases even lower), and 140.
The Welfare Index is taking the lowest value of several other indexes:
The Health Index
The Education Index
The Transportation Index
The Supply Index + 41
The Employment Index + 37
The Social Security Index + 38
The Government Salary Index + 27
The Consumption Index contributes between 0 and 8 point to the welfare index. It needs to be at a level of about 300, to reach the max effect.
The game level will increase the welfare index by 1 point, for each game level higher than 10.