I calculate about 58 IBUs for 5 gallons, assuming the Cluster alpha at about 7%, Northern Brewer at 9%, and Centennial at 10.5%. Here's how I swag it, for 5 gallons -- it's all about memorizing the magic factors at the beginning:
3.6 * oz * AA% = IBUs from bittering additions @60 minutes
1.6 * oz * AA% = IBUs from flavor additions @10-15 minutes
0.6 * oz * AA% = IBUs from aroma additions @5 minutes
Add all these together, then add about another +1.5 to the final total and round everything up.
So I get about 50.5 + 3 + 3 + 1.5 = 58 IBUs. Nowhere near the 90s, like you said.