Ibu Calculator Additions

When trying to figure out the ibu's in by beer using a calcualtor like the one here: http://www.rooftopbrew.net/ibu.php I'm entering the times for each addition of hops as the total time that those hops spent in the kettle.

Is this the general consensus or should I enter the time that that specific addition spent in the kettle until the next addition of hops. I assume the formula is meant to use total time but started to doubt myself when my beers seemed to be less bitter than the ibu calculation said they should be.

Thanks in advance,
Dan

The time should be the total time of the boil with the hops in it.

Most recipes call for a 60 minute hop addition. At that time, I set my timer at 60 minutes and then the clock goes "backwards" to the 20 minute hops (with 20 minutes left in the boil) to the 5 minute hops, and then flame out when the timer hits "0".

So, the 60 minute hops boil for a total of 60 minutes, the 20 minute hops are in the boil for 20 minutes, and so on.