The rule of 18 does not work with common fret scale calculators or rulers. I have all the Stew Mac rulers and use the Stew Mac fret position calculator which uses the modern 12th fret measurement. I have measured numerous vintage guitars, most recently a 1962 SG Standard, and they all line up perfectly with one of the Stew Mac rulers. I do have the rule of 18 scale on some of my templates that I sell but I have not yet measured a guitar that uses that particular scale. Regards Peter.