how i decide on rating: i firstly roughly rate every drama out of 10 as a whole, and i then come up w a separate set of ratings based on 2 criteria - 1) how good the plot is (consistency, pacing, how interesting etc) and 2) overall atmosphere of the drama (feelings experienced while watching, be it due to the scenes, attachment to characters or elements such as colour grading, set design, music etc).
the average score of the 2 criteria is then taken and compared against the general rating. for dramas that have differences of 0.5 and above in my two ratings, the final rating will be re-evaluated and adjusted accordingly. however, for a small proportion of dramas, the final rating may differ by 0.5 from the average rating due to various circumstances, such as presence of major flaw which affected watching experience (and hence lowered score).