im not sure what you mean by gauge, when building my grills and working with any other armour item that has an opening restriction i use an old socket that measures 7/8th of an inch, not being able to push the socket through an opening tells me ive done it right
There is a template for one in the SCA Fighter's Handbook. It might need some tweaking for current standards and various Kingdom standards. It is also available here http://www.tssinc.net/~daniel/gauge.html .