This isnt a simple technique to me
I made some Oragami boxes with belly bands and the only way I could make a belly band to fit was to just cut the strip of cardstock and lay the box on it and wrap the cardstock strip around it and glue it together. I tried measuring and figuring but that was all too much for me! I even tried to measure it after I already folded it around the box and that didnt work!
I am belly band impaired;)