Alternative Method:

What you’ll need: 
Your Binos, Paracord, Tape Measure, Marker

Step 1: 
Wrap the paracord tightly around the binoculars where you want the clamp to sit, preferably as close to the center as possible. Use your market to mark the location where the paracord wraps around tightly to make a complete circle.

Step 2: 
Use your tape measure to determine the circumference of where your NS will mount and compare it to the chart to get your size.