Bob Wescott, the author of “The Every Computer Performance Book,” discusses the importance of accurate data and notes when it is acceptable to use less accurate measurments. He also explains how to best make estimates regarding performance expectations and peak loads:
If the numbers show a resource will be 20-25% busy at peak, I would not spend more time getting a more precise version of that number. You could be off by a factor of two and the resource would most likely be fine at 40-50% busy. Obviously, the closer you are to some performance limit, the more careful you have to be with your calculations, models and predictions.