But note, don't confuse a "rule of thumb" with a mantra (my next posting). Rules of thumb are based on calculations and math. Matras are based on wishful thinking.
And so forth....
There are of course, a lot more. For example, see these sites:
The point of a "rule of thumb" is to take complex economic equations and reduce them to simple terms than you and I can use on a daily basis without resorting to calculus. And people use these in all walks of life. A contractor can look at a set of blueprints and using a "rule of thumb" based on square footage tell you the cost of construction in a matter of minutes - and how many board-feet of lumber are needed.
Your doctor can measure your height and weight and have a good guesstimate of your body mass index, without you having to strip down to your skivvies and sit in a tank of water. Rules of thumb help us by taking complex data and reducing it to a simple equation.
They are, of course, no substitute for "doing the math" on any topic. If the "rule of thumb" shows that something is promising, then you can get out your pencil and see if it really will work out. For example, in renting out a condo or house, I could guess pretty accurately whether I would make money on the deal by using "rules of thumb" to guess at mortgage, tax, and insurance costs. But of course, later on, I would get out my calculator or spreadsheet and go to town on the numbers to see exactly what I could budget for the project.
Some of these "rules of thumb" are numerically based such as the 50/30/20 rule, others are basic good advice (e.g., "Never co-sign a loan!"). The point is, there is some logic behind these "rules" that you can analyze and see if it fits your situation.
In most cases it does. Rarely does it not. There are few, if any circumstances where saving money is a bad idea and spending it is a good one.
But that doesn't stop the marketers from coming up with their own "rules of thumb" - or mantras as I call them, which is the subject for the next posting.