The English phrase rule of thumb refers to a principle with broad application that is not intended rule of thumb means a "rough measurement". He says that "Ladies often measure yard lengths by their thumb. Indeed, the expression 'sixteen.
As a general rule of thumb, the length of the rode (anchor line chain) should be how many times the depth of water in which it will anchor.
As a general rule of thumb, your rode should be 7 to 10 times the depth of the water in which you will anchor. You will need more rode in bad weather or rough.