I ran across this gauge the other day on Etsy — it “holds constant the Golden Ratio and takes the math out of creating pleasingly proportioned Art and Craft items.”
If you don’t know about the golden ratio (sometimes called golden section), from Wikipedia:
At least since the 20th century, many artists and architects have proportioned their works to approximate the golden ratio—especially in the form of the golden rectangle, in which the ratio of the longer side to the shorter is the golden ratio—believing this proportion to be aesthetically pleasing.
There are directions for making one yourself here, as well as a printable transparency or golden ratio ruler that “to see if its design is based on the golden section.”