calculate changes in "lines of vision"

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NSAspy

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I need some help figuring out how many lines someone's vision changed. I get that a change from 20/25 to 20/30 represents a drop by one line etc. I'm struggling with changes in between or beyond the graduations in the snellen chart, say 20/100 to 20/150, or 20/150 to 20/800.

I'll really appreciate your help, and yes i'm searched around. :)

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