A rule of thumb
Definition 1 of 1
Pro Tip 1/3Not Precise
The phrase implies a general approach and is not meant for situations requiring accuracy or scientific measures. 
A rule of thumb for cooking pasta is to use one liter of water per 100g of pasta, but adjust as needed.
Pro Tip 2/3Experience-Based
The guideline mentioned is often derived from someone's experience rather than official rules. 
A rule of thumb in business is to always follow up with clients within 24 hours.
Pro Tip 3/3Common Usage
It's widely used in everyday English conversations and is familiar to most native speakers. 
As a rule of thumb, fill your car's gas tank when it reaches a quarter full.