Nearly zero. That's part of the hypothesis: that it be the smallest possible unit of suffering. If the logarithmic scale of quantification for forms of suffering holds true, then forms of suffering at the maximal end of the scale would be practically infinite comparably.

As long as it's nonzero, then as I stated before, there exists some N such that N dust specks have greater negative utility than fifty years of torture. 3^^^3 and 50 are just proxies for whatever the true numbers are.

Correct, but a number that approaches infinity is not itself necessarily infinite; merely very large. 3^^^3 for example.

This is a category error. 3^^^3 does not approach infinity. It's a fixed number, it's not going anywhere.

The rest of your comment clarifies the offending inequality.