I used to add in last 20 minutes, but read somewhere on here that 15 minutes was better. I use a heaped teaspoon per 22 litres. Recently I started pre-soaking in hot water and cutting the particles with scissors. I think - but am not certain, that clarity of finished beer has improved. Would like some of you to share your kettle finings details, and expect that some of you use other finings, or none.