you should specify the fill-factor for an index based on the number of inserts,
updates on the index key(s)
Not only updates on the index key can cause fragmentation
the more inserts on the index the less the fill-factor should be
Wrong again. E.g. an insert into an ever-increasing index will never lead to fragmentation. Or an insert into an index with several static key values won't create fragmentation either.
My advice is if one doesn't fully understand what is fill-factor and the patterns of updating data in the given database then it's better to leave fill-factor to default value, otherwise the harm may outweigh all the benefits.