I dunno what version of beersmith you're using, but mine (2.0) is giving me 44.3 IBUs for FWH and 40.3 IBUs for 60min Boil, that's for 20g of 6.4% Cascade in a 10L batch.
You are right, my memory was faulty. it does +10% for FWH and -80% for mash hops.
Primary: Revelation Cream Ale
On Tap: Red Skies at Night, LHB Blond Ale, 224? Suburban Wheat Ale
What happens if you steep your FWH at 170 for 30 mins, but remove them before hitting a boil? Any detriment to bitterness?