No, there is no "default C". The problem is the rankings need to be fitted on a curve. So there needs to be "enough people getting C". That's why for freshly promoted officers who are good, but are not super duper outstanding in the year after their promotion, we let them fill the quota for average performers.

However, there are people who continue to shine so brightly and outstandingly and they do get better performance grades regardless of when they were last promoted. What I've seen is.. the most common kind are high performing BTs (regardless of scholar status) who work very hard and smart; they contribute at a very high level as a freshly promoted GEO3 and can really outshine the more lao jiao GEO3s and consequently get C+ and B even in the year they get their GEO3.