Take a look at this guy's proportion measurements I found on reddit

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He's 6'2" and his sitting height is 78cm.

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idk why but I can't post the screenshot

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Here's the link
https://www.google.com/amp/s/amp.reddit.com/r/tall/comments/61dgys/whats_your_sitting_height_and_buttknee_length/

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I doubt the 6+ guys who measured a low sitting height did it right, or they probably are just very skinny which affects sitting height. In my opinion sitting height is not a good measure of leg length, the best way should be using the height of your belly button compared to total height, because it can't be affected by fat or improper instruments. Its calculated as height/belly button height and the proper ratio is 1.68. 

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You mean the golden ratio:
Φ = 1,618

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Yes, but with my formula 1.68 is the equivalent of the golden ratio.

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What's the lowest ratio to look proportionate?

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1.65. This would roughly translate to a 48% inseam ratio.