How acceptable is it to ask a professor a question such as "so you think it's justifiable to take points off my proof for this?"? TLDR: Professor apparently only takes at most one look at a student's answer to determine correctness, doesn't award full marks kind of unfairly (clarifications below), is offended by hearing the question in the title, and grade reviews end up taking up to 5 hours due to complications I try to make clear here:

Last Thursday, our professor lets us know he'll be in his office doing grade reviews for everyone so that we have a fair grade, see if everyone agrees with his correction and all that stuff: I'm the first one in and I start reviewing my answers to the first and easiest items on the exam... and I find that the professor could have been a little fairer.

After disagreeing with a few marks taken off because of small stuff, I ask:

- Then you think it's justifiable to take points out of the exam because of things as small as this?

- Watch your tone! If you want a 10 right now, just walk out of my office right now, don't speak to me again and I'll give it to you!

And then, before I even had a chance to say anything at all, he crossed out my 8.2 and wrote a gigantic 10 where the 8.2 was before. When I had a chance to say my piece, all I did was apologize and try to emphasize as much I possibly could that I meant no disrespect at all, I was just trying to gauge how rigorous of a standard I should expect so that any mistake I made in the past wouldn't be repeated.

After a while of my apologies, he said:

"Alright, forget about it, let's just start over and pretend it never happened"

A while after that, I was still pretty anxious, so he gave me some time to calm down and my grade went from 8.2 to 9.6. I guess what I'm looking for in an answer is advice on what I could have done better so that I don't mess up like this in the future.


  Some helpful background but not absolutely necessary: A colleague of mine had an initial grade of 9.3, after the review, it was 9.9. The reason was that in his answer to a problem asking for a proof of a certain statement, he actually stated and proved a much more general result and said the initial statement was just a particular consequence of that result - the professor didn't like that and thought a 10 was undeserved. Another colleague mentioned a story of grades going up so far as 9.5 after the review (with an initial grade of 1.5) in another exam of some other course he gave on the past. Yet another colleague shared that his initial grade was 3.8, but he managed to convince the professor that a lot of his other arguments were right too - his grade would then go up to something between 5.5 (the bare minimum for a C is 5) and 7, and in order to avoid that the professor took points of another unrelated answer which had initially been awarded a full mark, said "alright, you got a 5, it's good enough" and left it at that (my colleague was the first one after me in the office and heard what had happened before because the door to his office was open, so that's why I think he didn't argue anymore). Finally, I and another colleague whose grade went from 8.3 to 9.3 agreed that a general impression of his correction was that he just read your proof once and if he didn't agree that was it (where it also happened to him that he proved more general results than asked and the professor didn't like it or hadn't read the arguments carefully). All in all, the review started at 2 pm and ended some 4-5 hours later and a lot of people got their grades up.
It's quite difficult to catch the tone of what you said from a written recollection, but:


  So you think it's justifiable to take points of my proof for this?


(emphasis how I imagine the professor perceived it) is indeed not very respectful. Next time, try something like this:


  I'm sorry, I understand that I should have given a more detailed reasoning about why [...] but it's just such a small and obvious part of the proof -- isn't it a bit harsh to take off Y points for this, what do you think?


Keep in mind that if you do this often you might still get a reputation as "that kid who tries to argue every tiny point even when they already have a pretty good grade" but if you are reasonable about getting told "no" it shouldn't be much of a problem. Still, you have to weigh your chances of getting an increased grade against the chances of being perceived as annoying or needy.

However, the professor's response in this case is completely unacceptable. Giving you full marks based not on your performance on the test but based on you getting out of their office is a gross violation of academic standards (even if they later rescinded this "offer"). Honestly, the whole thing that transpired here sounds like both sides messed up. It sounds like the professor (rightfully) immediately regretted "offering" you full marks.

So many people getting their grades increased sounds like the professor should invest a bit more time in designing a clear grading model and then consistently apply it. The way they currently have it set up pretty much guarantees having to argue every little point with every student. Even if what you said is inconsiderate and perhaps rude, they had themselves set up for it.