I'd also recommend some of the books by Neil Gunther at http://www.perfdynamics.com/ http://www.perfdynamics.com/, notably the PDQ one and Guerrilla capacity pl

I'd also recommend some of the books by Neil Gunther at http://www.perfdynamics.com/ http://www.perfdynamics.com/, notably the PDQ one and Guerrilla capacity planning. He takes queuing theory down to a slightly non-academic level and applies it to modelling computer systems such as web applications. From that you can fit observed data to your model and predict capacity on shorter cycles.