Using Rules of Thumb
Most of us use "rules of thumb" in all sorts of areas of our daily lives.
"When the needle on the fuel level indicator gets to the red, I know it'll last at least another 20 miles", for example, or "Software with a user interface that's anything other than grey or blue hardly ever gets through the user acceptance testing."
These rough rules, based on experience, are invaluable because they help us to make decisions without further detailed fact-finding: Drivers know they don't need to divert immediately to find a fuel station, and the software designer can save him or herself rework later by making the interface blue or grey in the first place.
But no one expects these rules to be 100% accurate: If the car ran out of fuel in 18 miles, the driver wouldn't be particularly surprised, and if testing showed that users actually liked an application that had a tasteful green interface, the developer might raise an eyebrow, but that would be all.
This is because rules of thumb are only used in situations where the risks associated with using a "good enough" approximation are acceptable. No one would use a rule of thumb such as this fuel usage one for a F1 racing car: They'd do a detailed analysis of the exact amount required to cover the race distance.
The formal term for these rules of thumb is "heuristics". Heuristics are a topic of interest in various fields from computer science to psychology and philosophy, but the principle is the same in all of these.
When to Use the Tool
Because a heuristic is a model which offers only a limited representation of reality, it should only be used when the speed or low cost of using it makes it better value than using a detailed set of data. So heuristics are often used in calculating estimates, for proof-of-concept prototypes, or for filtering a set of options so that you are left with a shortlist of a manageable size.
Constructing a Heuristic
Heuristics need to be formalized if they are to be most useful. This raises them above the level of "gut instinct", and it also means that they can be shared with other people.
When you find yourself using your experience to make a judgment, try to work out the rule of thumb that you've just invoked. If possible, add this to your procedure manual, or talk about it with a colleague so that your experience is validated and passed on.
A typical example in a car breakdown call centre might be...