rule of thumb vs specification

rule of thumb

noun
  • Approximated, guesstimated. 

  • A general guideline, rather than a strict rule; an approximate measure or means of reckoning based on experience or common knowledge. 

specification

noun
  • An act of specifying. 

  • An explicit set of requirements to be satisfied by a material, product, or service. 

How often have the words rule of thumb and specification occurred in a corpus of books? (source: Google Ngram Viewer )