"calculated IBU's" are based upon hop utilization estimators. A hop utilization estimator that is often discussed is the one developed by Glenn Tinseth but there are others (e.g., Rager, Daniels, etc.). At the recent National Homebrewers Conference there was a presentation on the topic of Scaling Hops that involved beers brewed by Columbus Brewing Company and a homebrewer. In that ‘study’ it was found that Tinseth, Rager and Beersmith underestimated the IBUs as compared to the measured values. In this particular instance the hop utilization estimators provided a worse indication of IBUs. I have no idea whether this necessarily also means a worse indication of perceived bitterness since there is no means to quantify perceived bitterness. Cheers!