The phrase "rule of thumb" is an idiomatic expression that refers to a widely accepted principle or guideline based on experience and practice rather than theory. Full definition
Generally, a good rule of thumb for older babies and children is to dress in one more layer than what an adult would need for adequate warmth. (blogs.uwhealth.org)
The general rule of thumb when dealing with matters related to layering in winter is that the baby should be dressed in one extra layer above what the adults have used. (pickbabystroller.com)