How to measure arm length correctly

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Hello, I'm trying to calculate my standart deviation of arm length according to this article

https://royalsocietypublishing.org/doi/pdf/10.1098/rsos.171790 (PAGE 5 AT THE BOTTOM)

And I don't know if I should measure my arm length from my shoulders to my middle finger or to the wrist, what's the correct way of measurement?

Best Regards

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Can someone enlighten me? I've read several old posts about arm length of the users, someone said that his dad was 187cm and his arms were 78cm, 78cm is impossible to have unless you count the middle finger. Is that the way how you get the arm length? I thought it was measured from shoulders to your wrist.