A rule of thumb is a general principle developed through experiential rather than scientific means.
A principle that comes from the wisdom of experience, usually but not always valid.
A rough, practical method of procedure.
1. In fashion, the rule of thumb is that you don’t wear white after Labor Day.
2. A good rule of thumb is not to purchase a car you can’t pay back within three years.
3. On dates, the rule of thumb is that the man should pay for dinner on the first date.
4. Here’s a rule of thumb when you’re buying a home, you should put 50% of your income towards housing and bills.
criterion, hit and miss, pragmatism, rules, tips, trial and error, unwritten rule