There has been SO much discussion and disagreement about how much Whirlfoc to use and when to use it, I did something that apparently few people have done...I contacted a technical person at the company! Yeah, I know, a pretty radical solution. Here's the reply....
"As a general rule of thumb 1 tablet will treat 15 - 25 litres of wort depending on the grist composition. The tablets should be added to the kettle 10 minutes before the end of the boil"