I do not believe anyone has mentioned the Bureau of Standards master gauge. It is improrant and bears discussion. For all the research that I have done, it has been in use for a very long time. It is available to most folks and quite handy. Of, course you all know it and that is your thumb. It is handy and mostly accurate. Think back, many of you have used this gauge. Why do you think that they call it "a rule of thumb" Thank you for your time and attention to the obvious.
Mike

USAF RET 1971-95 [Linked Image from jpgbox.com]