I'm guessing this is a Tableau Desktop question even though it somehow got categorized as a Tableau Prep question. Assuming that, I would suggest creating a calculated field with code like this:
SUM([Numerator]) / SUM([Denominator])
As it uses an aggregation, the calculation is called an aggregate calculation and will be recomputed every time you change the level of detail in a view. So, if you slice the data only by the Doctor, you'll get the results you described above.
Hope that helps!