How Do I Calculate the Number of Concurrent Users?

I have this old thing that we used, that the three of us used. We were looking at where the average think time, how much time a person might stay on a given page. And so this is a way of helping me describe the state and the stress that my application or system is under. So what if I said, okay, because this all communitive since it's multiplication, so page per second divided by CPU, that answer times average think time,. That would be our number of concurrent users that we were estimated we could handle.