It is probabbly from the time when the first Mil C-45662 concerning calibration control was written. Today the field measurement devices are becoming exceedingly accurate therefore the backwards traceability to the working standards, reference standards, and Master Stndards or National standards is not able to maintain this ratio of 1 to 10. Therefore in many of the new standard guides you will see  a mention of a ratio of at least 3 to 4 times or better. It is for the user to decide. For a more thorough examination you may have to evaluate the system accuracy and the uncertainity aspects in line with the new standard guidlines mentioned in ISO 9001 and other associated calibration standards like ISO guide 170542. I am not sure of all the standard numbers, please check with any ISO reference table.

QS9000 , Technical manal , MSA

The "Gage Mentor Workbook" from Resource Engineering in Tolland, CT. www.reseng.com

I have been trying to convince management where I work to get to 4x.  I feel comfortable with that number based on our tolerances (+/- .030 in most cases).  When it gets down to the tighter work I think the number is still accecptable becasue the talent levels increase as does the proven ability to hold tighter tolerances with Mics and bore mics and high end hand measuring tools.

Fill what's empty. Empty what's full. And scratch where it itches.