We do Guesstimations (calculated or not) once in a while… and this is an interesting read about it…
http://3quarksdaily.blogs.com/3quarksdaily/2008/09/useless-calcula.html
In the Oracle side… we also use this back-of-the-envelope-calculations for coming up with man-days, capacity planning, hardware recommendations, or just simply knowing what’s happening. And I like these phrases from the article…
“But how can we know the actual figure? We cannot. We can only get closer and closer approximations by measuring things more and more accurately (the volume, not just of the building, but of everything in it, which must be subtracted). It’s not like there’s an easy way to pour the air out of the building and weigh it!
The fun in doing these estimates is in NOT looking anything up, and instead trying to answer questions by using, along the way, what we do know to estimate everything we need to know to answer our question.”
Which means if you are building a mission critical system.. the goodness of the guesstimates is determined by the quality of the sizing requirements and how detailed it could be. And it is impossible to build good systems without this information which may result in being oversized (money loss) or undersized (customer loss).
I also like this comment from Greg Rahn (http://goo.gl/gFNO)
“The problem I see with IT shops and planning is there is there is either no planning, or it is done incorrectly. Very few shops take the time to build and test, it is more like build and deploy, then “tune”. Let me put this into a slightly different context: If you were tasked with building a bridge from point A to point B what information would you need? Would things like amount of traffic, both peak and average, type of traffic: train, bus, trucks, cars, bicycle & foot be important? It would to me. I see IT shops that design a foot bridge (because it is cheaper) and then try and put cars onto it and then it falls down and they complain. Capacity planning and building computer systems is not about guessing, it is about engineering and calculations. That being said, what calculations go into your system design?”
How about you? Do you have any interesting Oracle Guesstimations? 
