We obviously have a different view on the subjectivity of morals, no doubt an argument that's been had many times before. The sequences claim to have resolved it or something, but in such a way that we both still seem to see our views as consistent with them.
To me, subjective morals like you talk about clearly exist, but I don't see them as interesting in their own right. They're just preferences people have about other people's business. Interesting for the reasons any preference is interesting but no different.
The fundamental requirement for objective morals is simply that one (potential future) state of the world can be objectively better or worse than another. What constitutes "better" and "worse" being an important and difficult question of course, but still an objective one. I would call the negation, the idea that every possible state of the world is equally as good as any other, moral nihilism.
I accept that it's used for the subjective type as well, but personally I save the use of the word "moral" for the objective type. The actual pursuit of a better state of the world irrespective our own personal preferences. I see objectivity as what separates morals from preferences in the first place - the core of taking a moral action is that your purpose is the good of others, or more generally the world around you, rather than yourself. I don't agree that people having moral debates are simply comparing their subjective views (which sounds to me like "Gosh, you like fish? I like fish too!"), they're arguing because they think there is actually an objective answer to which of them is right and they want to find out who it is (well, actually usually they just want to show everyone that it's them, but you know what I mean).
This whole argument is actually off topic though. I think the point where things went wrong is where I answered the wrong question (though in my defence it was the one you asked). You asked how I determine what the number N is, but I never really even claimed to be able to do that in the first place. What I think you really wanted to know is how I define it. So I'll give you that. This isn't quite the perfect definition but it's a start.
Imagine you're outside space and time, and can see two worlds. One in which J is left alone, the other in which they're eradicated. Now, imagine you're going to chose one of these worlds in which you'll live the life of a then randomly-chosen person. Once you make the decision, your current preferences, personality, and so on will cease to exist and you'll just become that new random person. So, the question then becomes "Which world would you chose?". Or more to the point "For what value of N would you decide it's worth the risk of being eradicated as a J for the much higher chance of being a slightly happier N?".
The one that's "better" is the one that you would choose. Actually, more specifically it's the one that's the correct choice to make. I'd argue this correctness is objective, since the consequences of your choice are completely independent of anything about you. Note that although the connection to my view on morality is probably pretty clear, this definition doesn't use the word "moral" anywhere. The main post posits an objective question of which is better, and this is simply my attempt to give a reasonable definition of what they're asking.
This is based on a discussion in #lesswrong a few months back, and I am not sure how to resolve it.
Setup: suppose the world is populated by two groups of people, one just wants to be left alone (labeled Jews), the other group hates the first one with passion and want them dead (labeled Nazis). The second group is otherwise just as "good" as the first one (loves their relatives, their country and is known to be in general quite rational). They just can't help but hate the other guys (this condition is to forestall the objections like "Nazis ought to change their terminal values"). Maybe the shape of Jewish noses just creeps the hell out of them, or something. Let's just assume, for the sake of argument, that there is no changing that hatred.
Is it rational to exterminate the Jews to improve the Nazi's quality of life? Well, this seems like a silly question. Of course not! Now, what if there are many more Nazis than Jews? Is there a number large enough where exterminating Jews would be a net positive utility for the world? Umm... Not sure... I'd like to think that probably not, human life is sacred! What if some day their society invents immortality, then every death is like an extremely large (infinite?) negative utility!
Fine then, not exterminating. Just send them all to concentration camps, where they will suffer in misery and probably have a shorter lifespan than they would otherwise. This is not an ideal solutions from the Nazi point of view, but it makes them feel a little bit better. And now the utilities are unquestionably comparable, so if there are billions of Nazis and only a handful of Jews, the overall suffering decreases when the Jews are sent to the camps.
This logic is completely analogous to that in the dust specks vs torture discussions, only my "little XML labels", to quote Eliezer, make it more emotionally charged. Thus, if you are a utilitarian anti-specker, you ought to decide that, barring changing Nazi's terminal value of hating Jews, the rational behavior is to herd the Jews into concentration camps, or possibly even exterminate them, provided there are enough Nazi's in the world who benefit from it.
This is quite a repugnant conclusion, and I don't see a way of fixing it the way the original one is fixed (to paraphrase Eliezer, "only lives worth celebrating are worth creating").
EDIT: Thanks to CronoDAS for pointing out that this is known as the 1000 Sadists problem. Once I had this term, I found that lukeprog has mentioned it on his old blog.