The biggest problem with percentages is if you read nonfiction. The back matter is included even though notes and indices won't be read through. I'd rather calculate my own percentage; with ADE, I look to see on what page the notes start and do the arithmetic as I read.
But I have to say, that at 50% I'll have no idea how long it took me to get to that point. However, it's easy for me to relate my reading speed to ADE pages.
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