Individual perceptions aside:
When selecting FWH in BeerSmith, I believe it uses the boil length you've entered in the recipe to calculate contributed IBU's. If I recall, changing the steep time (i.e. 15 min, 75 min, etc) within the actual FWH step doesn't change the IBU contribution.
That being said: I just brewed my SMASH hoppy lager. For 10 gallons and a 75 minute boil, I use 3 oz. of FWH to get about 31 IBUs (5.4% US Saaz), and 2 oz. at 15 minutes left, to get another 12 ibu. This beer usually has a nice smooth bitterness, with plenty of flavor and aroma.