There is no such thing as an accurate, global freespace percentage. Freespace needs to be set index by index. Thus, a generic rebuild should never specify a fillfactor, but let it default to what has already been set for that index.
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial: "If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them."