I'm sometimes asked which perforation gauge I use. In my opinion, the hands down choice is the Precision U.S. Specialty Multi-Gauge made by Sonic Imagery Labs.
There are perforation gauges available for as little as a $1 or $2. There is nothing wrong with them.They are accurate enough for most stamps.
However, the Specialty gauge is 11 measuring devices in one. Did you know that there are actually four different sizes of gauge 11 perfs? The differences are tiny. But when identifying early US stamps, they can be an aid in detecting reperforated stamps. The gauge includes grill patterns for grilled stamps as well as measurements for rotary versus flat printed stamps. The gauge is very comprehensive. I use mine all of the time.
The only downside is that the gauge is big. It's about 3 inches wide and 11 inches long. That sometimes makes it a little cumbersome to use. The gauge costs about $15 which is a little pricey compared to cheaper gauges. But its abilities make it worth every penny.