How to Shorten Men-Himo
The men-himo should be no longer than 40 cm (about 15.5 inches) from the knot behind your head to the end of the himo when it's tied, according to International Kendo Federation (FIK) regulations. Having the proper men-himo length also allows you to tied your men-himo faster.
Standard dojo etiquette disclaimer: If your sensei teaches you a different way, please follow your sensei's instructions.