Pay no attention to the wizard behind the curtain!
As a teacher that has just printed out 88 grade reports for the first quarter, I regret to inform you that it is almost always a judgement call, that benchmarks are rarely well written, and that each teacher has their own ideas about how to assess whether a student meets the benchmarks.
I had 6 benchmarks for this quarter. I designed a few assignments to teach and assess these benchmarks. I look at the grades I've given for each benchmark, and ask myself, "Do I have evidence for proficiency?" I weight some assignments more heavily than others. Then I look at the grades for the benchmarks. If there is Proficient for all of the benchmarks, the student gets a B. Most Proficient or Partially Proficient gets a C. Some benchmarks Proficient gets a D.
It seems like an odd system, but it has more meaning for me than the points and averages I had growing up.