The 7⁄16-inch button is the largest of the five available diameters. Total button length is determined by measuring from the bottom of the spring (in this case, the bottom of the spring slot) to the underside of the head. To get the spring height, subtract the thickness of the material you are fastening—in this case, the fiberglass—plus any additional clearance needed between the head of the button and the tab. There is a recommended deflection for each spring height and button-length combination. There are reference charts online (we used DzusFasteners.net) to determine the proper fastener for each application.