How does Clonazepam work for you?

I have severe anxiety disorder, mainly when it comes to public speaking. Earlier today, my psychiatrist prescribed me Clonazepam (Rivotril) 2mg & instructed me to take only 1/4 of it. I have an upcoming presentation next week, and I'd like to know how it has worked for those who have taken this medication? Also, how early before the presentation or triggering event do you usually take it?