So with several of the other users that Eugine had hit, the difference between his downvote total and that of the second-highest downvoter was quite drastic: in one case, there were 26 times as many downvotes from Eugine as from the second highest downvoter.

The pattern is different in your case: the top ten downvote balances against your account are 150, 74, 55, 36, 32, 31, 28, 20, 19, 17. (Eugine doesn't appear to have hit you, as he isn't included in this list.) It's plausible that the 150 person is a mass downvoter, and also that the 74 person is, given that the 74 person also had a suspiciously high downvote count towards another person. But at the same time, it also looks like there were a lot of people downvoting your comments. If I assume that most of the users in this list were "legitimate" downvoters, then I'm unsure of whether this data alone is sufficient to indicate exactly who the mass downvoter(s) was. The 150 person is the most likely culprit, but maybe it was several of the lower-ranking ones acting independently from each other, and the 150 one just happened to see a lot of your comments that he didn't naturally like? Whose downvotes should I have reversed, and whose should I let stand?

Then again, I don't know how large of a fraction 150 comments is of your total comment history: if it's a high percentage, then then it sounds more plausible that the person in question is indeed a mass-downvoter, since it would be much more unlikely for them to run into 150 of your comments that they just naturally disliked.

And now I have the feeling that the rational course of action would be to pick some percentage where I'd act as if this was a confirmed mass-downvoter, before hearing the answer to "what percentage of your comments is this"... but I don't have a very good clue of where I should set the burden of evidence in cases where the situation isn't blazingly obvious.