For our rough estimates, we are assuming the following:
All clients are Web clients.
Therefore, the only inputs to be used are: total number of users, and percent concurrency.
The average size calendar event size is 5K.
Each person creates ten events or todos per week.
80% CPU utilization.
900 MHz CPU's.
1 GB RAM per CPU.
Two years' worth of calendar data stored on system.
Six hot backup copies held in storage.