To be completely honest, the Acc% is a good way too gauge just in WHICH category the player is on this map, being those categories the ones Aqo posted.
Just to be a lil' bit more precise on the matter, S ranking starts at 95,0001 acc% (YES 95 is an A) A goes from 95 to 90,0001 (the same applies to any other rank) and then from this point on it decreases in values of 10, so B is 90-80, C is 80-70 and D is -70.
With this in mind let's get into the scores, this is a lil' bit ambiguous since the factor that decides the Rank is the ACC ONLY so... yeh, pretty much a guide to just know how much you need to get in a Medium note density map, not too much, not too little. Going into the too much the scores tend to get lower, meaning that the more notes you get the more are your chances to increase ACC even if you have a shitty score (Already confirmed it with an autoconvert of 30 mins.... Trust me.... a nightmare to get a decent score while trying to get a 95% acc or so to speak A just barely missing the S) and for the lower note density it just goes the other way around you tend to need higher scores for getting a higher rank.
So, Aqo pretty much did the job on this part but i'll put it again with a more accurate value (on mid-density maps I repeat)
D: Pretty much everything that oscilates into 0 and 400K
C: +400K pressing on the 515K mark (This is pretty much an estimate since its difficult to get a C on pourpose, at least for me XD)
B: +540K is definetily a B, then it kinda stops to fade out in the early 600
A: +600K and it kinda stops at 740K
S: +750K until 980K And I'm pretty sure its quite accurate, there are exceptions of course, but its pretty difficult to find an S in the 985K mark and impossible to see one on the 990's
SS: from 980K onwards..... not much to say here.
Having these data in mind I think that's better making an estimation of quality play on the Point department, but for minor adjustments on the point giving use the ACC % factor as a second option, in other words weight more the actual Score, but for scores pretty similar (tends to happen a lot) Value the ones that have more ACC%
I mean, come on, I've seen cases like of an S having (not really but something like this) 985.575 and an SS having 985.475 And man I'm sure that's just cause the guy who SS'ed couldn't get the timing good and failed at MAXing everything It is painful to have a perfect ACC and just be beaten because of the random point giving from the 300/300g as xcrypt said.
Also, I think that the Density of the map in mania affects a lot these aspects.... ACC and scores tend to be thrown off gaphics by HYPER density maps.... its just silly XDD
PS: Just trying to put some more numbers in there and also try to get the ACC and Score on the same post just to compare them more easily