Calculating a percentage between two tables

Hi all,

Trying to calculate a percentage between two tables with the below coding, and struggling to get an ouput. Wondered if anyone can point me in the direction of where I am going wrong?

I'm looking to generate a completion percentage per area, of staff completing a form. In essence two tables, first table ('Quiz Results') is the output of the form whereby staff are selecting the area they are based in during the form completion [Area]. Second table ('Staff Base') is a staff list with a field for the location the staff are based in [Base_Name].

AreaCompletion% = 
DIVIDE(
    CALCULATE(
        COUNTROWS('Quiz Results'),
        'Quiz Results'[Area] = "London"
    ),
    CALCULATE(
        COUNTROWS('Staff Base'),
        'Staff Base'[Base_Name] = "London"
    )
)

When applying to a card I'm just getting a blank output. Expected output would be 56.48%, as we have 431 responses with an area of "London" where there are 763 staff with a Base_Name of "London".

Thanks