Try studying a few recipes, especially ones you're familiar with. A recent IPA recipe I made has only 5 oz/ 5 gallons, with an OG of 1.079. It's just kegged but the hydro sample was quite hoppy. BeerSmith rated it at 121.7 IBU and 1.583 IBU/SG. Other recipes use 10oz and end up with similar numbers. It's all in timing of hop additions.
Anyway, reference a few recipes. Even if you haven't tried the specific beer you can still see a pattern in the quantity and types of hops used. As you and gcdowd have mentioned the generally accepted upper limit of IBU detectivity is around 100. Your personal tolerance may be different.
I guess my answer would be, the predicted numbers are great but they also make some assumptions (AA variance, hop utilization, etc). Keep that in mind and base recipes on your experience. Kyle