Be it Preliminary, Main Examination or Interview, many candidates look out for cut-off to make own strategy while preparing for Civil Service Examination.
It is highly hypothetical and in absence of any authentic data available particularly about Preliminary Examination, it is difficult to provide same accurate information.
Even for Main Examination (Written and Interview) a very few candidates share their marks in public domain which results in a very small sample size to illustrate accurate trends.
You should try to understand the psyche of UPSC which clearly spells that they are looking out for capable people with originality. The question papers clearly indicate that it is not bookish knowledge which is at test.