If there has never been anyone recorded to have used 100% of their brain capacity, how can 10% of the brain’s capacity be quantified?

I am sure we all have heard it at one time or another, that humans only use 10% of the brain’s capability, maybe slightly higher for MENSA quality people. If you were never able to reach the end, how can you know what percent you are in? For example, if you came upon a crevasse and you could not see the bottom, and dropped your binoculars and it landed about 15 ft. down the crack, how can someone say it fell in only 8% of the crevasse so you should be able to retrieve it. In order to know what 10% of what the brain can do, you have to know what 100% of the brain can do. How do they know the 10% they say we use is really 60% of what the brain is capable from a thinking, reasoning perspective?