Threads featured the buttonhole gauge in their November 2008 Issue. http://www.threadsmagazine.com/item/4276/five-must-have-sewing-tools : Buttonhole gauge simplifies positioning buttonholes. This exotic device, which costs around $15, looks complex but actually simplifies positioning and measuring buttonholes, pleats, tucks, or anything else needing to be evenly spaced. It's a great time-saver, because it eliminates the need for calculating and carefully measuring intervals. It's easy to use--simply mark the position of the top buttonhole or first pleat, for example, and stretch the gauge to fit.

Clotilde sells it for $20, as well as other sites online for a dollar or two less. Nancy's notion had it on sale today for $16. http://www.nancysnotions.com/product/supplies/rulers+&+templates/fashion/simflex+expanding+gauge.do

It's also called the Simflex. It measures space from 3/8" to 3 3/8". For greatest accuracy expand to the fullest length, then bring back to spacing you want. Expands to 24". Made in USA.