No, I don't think it is at all. Certainly, that's one of the considerations that is presently taken into account, and it seems to me it would make sense. But my starting point would be that a day of a sentence does not mean a day served in prison. A person who gets a 30-day sentence will not spend 30 days, regardless of the conditions in which the prison might be.
So if you're trying to equate this, the difficulty is that 99.8%--I'm not making up that figure, that's a calculated figure--of prisoners serve no more than two-thirds of their sentence. So for 99.8% of prisoners, the arithmetic is just wrong. That's all there is to it, because they're not going to serve a day for every day of their sentence. They're going to serve two-thirds of a day for every day of sentence.