I believe the TDKs come in only one length (24"). So if your man is around 5' 11" or taller, I'd say get one in his pants waist size. The TDKs do run about 1" large in the waist, and that might be to allow for shrinking when washed.
I'm only 5' 9" so I wear the TDK a little higher -- right at my navel. That is the measurement I used.
Going to try and finally get pics of me wearing mine later this week (took a while to find the right socks.) ;D
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