Richard Kiusalas created two precision perforation gauges in 1965, one for US stamps and one for Canadian stamps. I don't know enough about US stamps to say which classic issues get good usage out of the gauge - for Canadian stamps, the Small Queen issue had a large number of printings and the Kiusalas gauge can help differentiate among them.
The gauges show spacings accurate to the thousandth of an inch. The perforations are shown in the traditional round-it-up-or-down-to-the-nearest-0.5, and a second number showing spacing in thousandths of an inch. So, 12-66 is a perf. 12 with an exact spacing of .066" between perforation pins.
Note that paper can shrink over time, so the perforation spacing originally found on the stamp might look slightly different when measured now.
I don't have a US gauge - I found that picture on the internet. I do have a Canadian gauge. They're made out of aluminum.


Seeing as how they're now 45 years old, they can be tough to find. Sonic Imagery Labs has created a new perforation gauge that includes a replica of the Kiusalas US gauge.
http://www.slingshotvenus.com/stamp...tyPerf5.htmlRyan