Definition of "rule of thumb" []

• A useful principle having wide application but not intended to be strictly accurate or reliable in every situation. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Using the rough rule of thumb that one gets about a tenth of whatever you put into the boiler out as distillate, 3,000 gallons means this operation was producing 300 gallons of alcohol at a time, which would add up to 600 gallons of 100-proof white lightning."