DAX function to calculate Coverage

I have table with multiple columns, the ones of interest are course_id, session_id and total_score. There are multiple entries for each combination of course_id and session_id. What I need is a DAX function to calculate the coverage defined by count of unique combinations of course_id and session where total_score is not null divided by count of unique combinations of course_id and session. This is what I have but it is not returning right results

```

Evaluated Sessions Coverage =
DIVIDE(
countrows(SUMMARIZE(FILTER('evaluacion_endetus_merged', not(isblank(evaluacion_endetus_merged[total_score]))), evaluacion_endetus_merged[course_id],evaluacion_endetus_merged[session])),
countrows(SUMMARIZE('evaluacion_endetus_merged', evaluacion_endetus_merged[course_id],evaluacion_endetus_merged[session]))
)

```