A good rule of thumb

She has a green thumb

It's a good rule of thumb to get cheap seats on flights as early as possible.

A good rule of thumb is not to pick up or offer rides to strangers.

It’s hard to guess how smart the machines are, but a good rule of thumb is that they’re always smarter than you think.

It's a rule of thumb for me if I'm at second when he has the ball I try to go for three.