| 10.50 is the actual prescription. 20/** is an approximation. For instance if you were 20/200 you would need to be 20 feet from something that someone with perfect vision would be able to see from 200 feet. Get it? It's not anything a doctor relies on. As far as I know, there's no real conversion as it wouldn't be too unreliable. Why is that you need to know, please?
You could also ask your doctor what you tested as. They usually don't test above 20/200 which is legally blind, I would assume you tested above that. | 
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