I think in certain contexts it makes sense to think about the closeness of two quantities in terms of percentage difference. For example, let's say we're not just talking about the numbers 98 and 100, but the rates 98 mph and 100 mph. When we talk about speed, what we're actually interested in is usually not the speed itself but rather the amount of time it takes to cover a certain distance when traveling that speed.
So in this context, it makes sense to say that 98 mph is about 100 mph to the same degree that 980 mph is about 1000 mph--because they have the same marginal relation in the time required to cover a certain distance at those speeds.
If you assign an epsilon of disutility to a dust speck, then 3^^^3 * epsilon is way more than 1 person suffering 50 years of torture.
This doesn't follow. Epsilon is by definition arbitrary, therefore I could say that I want it to be 1 / 4^^^4 if I want to.
If we accept Eliezer's proposition that the disutility of a dust speck is > 0, this doesn't prevent us from deciding that it is < epsilon when assigning a finite disutility to 50 years of torture.
I was thinking of it more like: if there's a certain place I can get to in (roughly) 102 hours going 98 mph, and I want to get there in 100 hours, I need to speed up to 100 mph. Similarly, if there's a another place that I can get to in roughly 102 hours going 980 mph, and I want to get to that place in 100 hours, I need to speed up to 1000 mph.
I kind of wanted to clarify that in the original post but I hadn't really thought of a good way to express it at the time.
Furthermore, I think that your interpretation of the example even makes it more clear that it makes sense to think of it in terms of a ratio. In the first case, you've sped up by 2 mph and gotten a gain of about 2 hours, straightforward enough. But in the second case, you've sped up by 20 mph, and only gotten a gain of about 0.2 hours. Here's where I think most people's intuition is probably screaming "whaaaaaaat!?"
But if we think of it in terms of the ratios, then everything fits together nicely again and the screaming intuition voice shuts up. Plus the math we have to do to get to the right answer is a lot easier.