After calculating the IBUs the way I calculate them the IBUs for this particular BW is ~74. The only hops I considered were the hops that were in boil for more than 10 minutes. In my experience those added with 10 min or less to go for the boil may contribute ~1 point.
This is the formula I use to arrive at IBUs - I beleive taken from JP.

Target = 8.5 AA x 1 (weight in oz) x .4 (% utilization of time (90min))
= 3.4 AAI (Alpha Acid Isomerization)
= 3.4 AAI / .01335 (constant #) / 5.5 (batch volume) = 46 IBUs

Fuggle = 3.3 AA x 1 (weight in oz) x .3 (% utilization of time (60min))
= .99 AAI (Alpha Acid Isomerization)
= .99 AAI / .01335 (constant #) / 5.5 (batch volume) = 14 IBUs

Goldings = 6.2 AA x 1 (weight in oz) x .15 (% utilization of time (30min))
= .93 AAI (Alpha Acid Isomerization)
= .93 AAI / .01335 (constant #) / 5.5 (batch volume) = 13 IBUs
Adding them all up you arrive at 73 add 1 for the rest of the hops added at 5 min and 0 for the dry hops and you get 74 IBUs - 14 points higher than the expected.
If applied to other recipes I am consistantly above their expected IBUs. I beleive it is because the isomerization rates used. However, the rates are an estimate and believe them to be very close to actual extraction rate of hop oils.
Thoughts?
- WW