Rule of thumb

The usual rule of thumb says that to calculate when an investment will double, divide 70 by the interest rate.

Here's a good rule of thumb: Too clever is dumb.

Since it is impossible to second-guess what a user will want to look up, there is a general lexicographical rule of thumb to err on the side of inclusion.

I made a quick, rule-of-thumb estimate of the manhours required for the job.