The other important thing to note (using your example) is that if you are seeing numbers that tell you that you will be maxing out an entire machine, your design had better be capable of running on 10 machines. When all the meat goes on the bones of the architecture, it’s a very distinct possibility that the deployment of your application will need at least 3 or 4 machines. Designing an application which wasn’t meant to scale beyond 1 machine in that scenario introduces considerable risk into the project.

But, determining the actual number of machines needed is something we can figure out much later in the development cycle. We need to validate the architecture first.

Using a factor of 2 for your margin of safety is still somewhat risky due to the fact that our numbers won’t be precisely known ahead of time. In my example above, if the users are clicking every 1.5 seconds instead of 3, we’ve doubled the peak load and eroded the margin of safety to 0. You don’t want to design to run at exactly peak capacity. An order of magnitude (x10) is really what you want to be safe.

Of course, there may be edge cases where a 10x margin of safety is too expensive, but that’s something we can tackle on a case by case basis. Our general rule of thumb is to stick with that 10x mark.

The point is to get yourself into the general ballpark–not to be precisely accurate. This exercise is useful as a step to reduce risk from your project. The margin of safety is critical for that since your numbers won’t be precise during esitmation.