Intuitively speaking broken arm and broken toe are comparable. Broken arm is worse, broken toe is still bad. I'd rather get a broken arm than torture for 50 years, or even torture for 1 day.
For sliding scale of severities: there's a very difficult to compute but intuitively satisfying emphasis that can be imposed so the scale can't slide. It's the idea of "bouncing back". If you can't bounce back from an action that imparts negative utility, it forms a distinct class of utilities. Compare broken arm with torn-off toe. Compare both of those to 50 years of torture.
P.S: If you're familiar with Taleb's idea of "antifragility", that's the notion I'm basing these on.
Broken arms vs toes: I agree that any number of broken toes wouldn't be better than a broken arm. But that's the point, these are _comparable_.
Incomparable breaks occur where you put the ellipses in your list. Torture for 40-50 years vs torture for 1 day is qualitatively distinct. I imagine a human being can bounce back from torture for 1 day, have scars but manage to prosper. That would be hellishly more difficult with torture for 40 years. We could count torture by day, 1-(365*40) and there would be a point of no return there. A duration of torture a person can't bounce back. It would depend on the person, what happens during and after etc, which is why it's not possible to compute that day. That doesn't mean we should ignore how humans work.
Here's the main beef I have with Dust Specks vs Torture: Statements like "1 million broken toes" or "3^^^3 dust specks" disregard human experience. That many dust specks on one person is torture. One on each is _practically nothing_. I'm simulating people experiencing these, and the result I arrive at is this; choose best outcome from (0 utils * 3^^^3) vs (-3^^^3 utils). This is easy to answer.
You may say "but 1 dust speck on a person isn't 0 utils, it's a very small negative utility" and yes, technically you're correct. But before doing the sum over people, take a look at the people. *Distribution matters.*
Humans don't work like linear sensory devices. Utility can't work linearly as well.