I disagree. Their way of doing the conversion is far more principled than the original commenter's. It's absolutely appropriate to do a percentage conversion

I disagree. Their way of doing the conversion is far more principled than the original commenter's. It's absolutely appropriate to do a percentage conversion based in Kelvin, the percentage has a fundamental meaning in terms of the actual increase in kinetic energy. You would get the same percentage if you did the calculation in Rankine because they're both 0-based temperature scales.