Right now I'm at "Diplomatic Immunity" where I have to go meet that lady back at her house (trying to be vague here to not give away spoilers, but
if you've done the quest you'll probably know).

So at what percent would you say I've completed the Main Quest?

Going by the tags list on steam, almost half way mission wise. Quests can become longer later tough.

Almost halfway done. About 45% maybe.

Why don't you want to finish it early? It doesn't make much of a difference. Anyway, to answer your question I don't know the percent but I'd say your about 1 third done. Give or take some. Don't even remember much of the MQ to tell the truth lol.

EDIT: Yea I'd agree with the two posts above me.

40% done, roughly.

You'll know when you get to the point of no return. It won't be a surprise.

Awesome, thanks for the quick replies.

I think I'll go take a look in the main cities for some quests first. I haven't done anything more than pass through in Markarth, Falkreath, Riften, and Dawnstar, so I think I need to explore some more!