I have a mathematical analogy.  In this analogy the C+ to B- numerological range starts at 77% and ends at 83%. The quality of this range is clearly above an average range of merit. Any essay with a grade that falls within this range should obtain a “borderline good status.” If this range of percentages is considered the grey area for reviews then the critics who take a rigid stance with these numbers are being  disingenuous...
		
		
	 
RT has a few issues vis-à-vis interpreting an individual critic’s rating system and translating it into their own.
One scenario:  apparently, in borderline cases, a critic can tell RT how (e.g.) their 2.5/4 rating should be recorded.  Sometimes, this can be “fresh,” sometimes it’s “rotten.”  It depends on the movie and, I guess, the critic’s mood.  (Note that 2.5/4 = 62.5% - which is 
above the rotten/fresh threshold.  So, really, there should be no need for translation or clarification; 62.5% is fresh.  If the critic wanted his score to be recorded as rotten, it’d be simple enough to rate it 2/4 in the first place.)
Another: the “letter grade” system you mentioned.  Technically, the only letter that indicates “fail” is F.  Everything else (from D- up to A+) indicates 60% or higher - a “pass.”  Obviously, this is a poor system to evaluate movies inasmuch as it might be useful to know just 
how bad a movie is; and there can be a big difference between a 59% F= fail/rotten and a 10% F = fail/rotten.  Thus, it’s more-or-less assumed that (along with F)  D-, D, D+ are also bad/failing scores (different degrees of rotten).  But how far up the scale do these assumptions apply?  At least one reviewer gave 
Aquaman a C+ grade - which was duly recorded as rotten.  Typically, a C+ is in the 75-79% range. 

  Bottom line:  as applied to movies, the “letter grade” system is flawed and/or confusing.