Divide by applicable values

Check this. You can replace _parameterNotNA by _parameter if you want to see also NA vs YesNo in _current part of code.

Applicable dufoq3 = 
VAR _parameter = SELECTEDVALUE('Table Test'[Q1A Parameter Checked])
VAR _parameterNotNA = IF(_parameter <> "NA", _parameter)

VAR _current = 
    CALCULATE(
        //SUM('Table Test'[Q1 score]),
        COUNTROWS('Table Test'),
        KEEPFILTERS('Table Test'[Q1A Parameter Checked] = _parameterNotNA)
    )

VAR _yesNo = 
    CALCULATE(
        //SUM('Table Test'[Q1 score]),
        COUNTROWS('Table Test'),
        'Table Test'[Q1A Parameter Checked] IN {"yes", "no"}
    )

VAR _result = DIVIDE(_current, _yesNo)

RETURN _result