This here says that the range of the normalised values are 120 to 38, so to get a percentage, you have to map 38-120 to 0-100. However, if we look at smartctl
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So, it seems that 38 is the minimum it can reach, 50 is the failing threshold, and 120 is the maximum. Doing some terrible math, that comes out at ~15% integrity being 'failure'.