Grouped Z-Score using dimension from another table

Hi,

These are two of my tables joined on QuestionID:

I want to calculate a Z-Score using the average of all questions within each [Phys Content Area 1] for each student. 

I can get the mean for the student per [Phys Content Area 1]:

CALCULATE(AVERAGE(fact_results[ResultPercentage]), ALLEXCEPT(dim_question, dim_question[Phys Content Area 1]))

I can get the mean for the cohort per [Phys Content Area 1]:

CALCULATE(AVERAGE(fact_results[ResultPercentage]), ALLEXCEPT(dim_question, dim_question[Phys Content Area 1]), ALLEXCEPT(fact_results, dim_question[Phys Content Area 1]))

But calculating the standard deviation is tricker because it first needs to average the results for each student per [Phys Content Area 1] before calculating the standard deviation. CHATGPT gives me this (which I have tried fixing), but I get 'Parameter is not the correct type.'

 Thank you so much.