Rules of thumb

Typography

The optimal line length for readability is around 10-12 words (source: The Thames and Hudson Manual of Typography by Ruari McLean)

Telecoms and networking

The ratio of peak load to average load in a service with diurnal activity variations is approximately 3 to 1. Source: my own empirical observation from Wanadoo access logs and France Telecom telephone call usage logs.

Probability of a Web page having X incoming links referring to it: P = X ^ -2.1 Source: http://www.almaden.ibm.com/cs/k53/www9.final/

When specifying computers, for balanced performance provision one gigabyte of RAM per gigahertz per core/thread.

Any standard making use of ASN.1 is a piece of junk.

Photography

For most lenses, optimal sharpness is reached around f/8.

Camera light meters are calibrated for 12% gray. Common gray cards are 18% gray, so if you use one for metering, you should open up one half stop to compensate. Source: http://www.bythom.com/graycards.htm

The human eye is a 6-7 megapixel sensor. The monocular field of view is 180 °, the binocular field of view is 120-140°, and the normal focus of attention spans a 45° field of view.

Avoid Kodak products like the plague. Those products they make that are actually decent (i.e. the engineers managed to sneak them past the bean counters) soon get adulterated (like Tri-X) or discontinued (like PhotoCD or their medium-format digital backs). Prefer Fuji, Agfa or Ilford.