Capacity planning should never be sold as a guarantee that all will be well at the next peak. No matter how good a performance person you are, you can’t offer that guarantee. 
Why? You cannot prove a negative. For example, you can’t prove there are no monsters waiting to get you while you sleep because no matter how carefully you check, you might overlook some spot (like the closet) where they are hiding.
Liebig’s Law clearly shows that even a small and obscure part of the transaction path can become a major bottleneck if given enough work to do.
Capacity planning is more like a pre-trip checklist to ensure you have what you need, and all systems on this list are good-to-go. Invariably, you will go on that trip, and somewhere along the way you’ll discover you forgot X, don’t have enough Y, and for the first time ever you need Z. That’s all bad news, but remember that your capacity planning effort found bottlenecks that would have limited your throughput even more.
Even if you use load testing to add to your confidence, no load test is perfect, so you still can’t honestly guarantee a trouble-free peak.
So, do capacity planning to the best of your ability with the things you do know about, load test to the best of your abilities, but make no absolute promises. If you get caught short on some resource, take the time before the next big peak to learn about that resource and to do a more complete plan next time. Unless this is the last peak before you retire, you need to think long-term.
The Every Computer Performance Blog 