How Much Contingency Planning Is Enough?

I gave a web seminar with Adobe today. Following all proper best practices, I was on the line a full half hour early. Checked my audio levels with the operator. Dialed in on a second backup telephone line in case my first line dropped. Had a copy of my slides in hardcopy in case my computer died. Made sure my moderator was ready to take over and help out if I lost connection. Checked that the phone call was properly streaming over VoIP for participants who wanted to listen on their computer spe