This setting cannot be changed later, but you can always create and populate a new index with a different setting.

This aids with scoring the different fields against one another, and also with how important each word is within a field. You should attempt to be as close as possible, but being off is not very deterimental.

For reference, the US Constiution "title" field uses One, and the "text" field uses 100 (Actually about 85).