This advice applies specifically to Mnemosyne, KhatzuMemo and SuperMemo, since they all share a 0 — >5 scoring system. It may well apply to other SRSes, too.
The “readings” are of course pronunciations for characters.
The “meaning” is the meaning+context+intent of the sentence. Of course, many sentences can have slightly different meanings in different contexts; you should only tackle one at a time — it will make life simpler for you. What I mean is: one sentence, one meaning. Fix a context. So “A bun in the oven”, cannot refer to both baking and pregnancy. Make one “card”/QA pair that’s about bakery, and a separate card about babies. Better yet, give extra context so there is less ambiguity. OK, I’m waffling about a problem that doesn’t often come up, but there it is as advice.
So, here is how I would and do score a sentence:
0: You had no idea whatsoever what the sentence meant, and/or you did not correctly reproduce readings for multiple characters.
1: You had an idea, but were completely wrong about what the sentence meant, and/or you did not correctly reproduce readings for multiple characters.
2: You just missed it. You were slightly off on the meaning OR slightly off on a reading. So, for Chinese-learners: if you give the right sound but the wrong tone, that counts as “slightly” off.
3: You got just got by on this one. You knew what the sentence meant. Maybe you even initially gave the wrong reading but then changed your mind to the right one. Or, gave the right one but changed your mind to the wrong one.
4: You knew exactly what the sentence meant. You read it out correctly with no mistakes in readings, BUT, even though you got it correct, you have/had some doubt. At no point did you give an incorrect reading for a character; you were just slow/stumbling a little and lacked some confidence in your answer. You didn’t instantly give the right reading for one or more characters — you had to think for a relatively long time.
5: You knew exactly what the sentence meant. You read it out quickly, confidently and clearly with all readings correct (no mistakes). This sentence was a cakewalk for you.
Anyway, that’s how I have scored and continue to score my sentences for both Japanese and Chinese. You are of course free to have variations of your own, let one hundred flowers bloom and all that; I don’t think there is a “right way” to do this, as such. As you advance, you might want to narrow your grading down to the part of the sentence that you were aiming to learn (since you advance gradually, almost every new sentence will only be partially new to you). So there you have it, a rough guideline.