Help creating a measure that counts rows

This depends slightly on whether you have the column as a Boolean or not, but try the following:

True Flag Count =
CALCULATE ( COUNTROWS ( 'Table' ), 'Table'[ClosedFlag] = TRUE )

False Flag Count =
CALCULATE ( COUNTROWS ( 'Table' ), 'Table'[ClosedFlag] = FALSE )

If the column is Text rather than Boolean, then try putting quotations "" around TRUE and FALSE.

To explain quickly, the CALCULATE wraps around a table expression that counts the rows in a filtered table. The first parameter is the table expression and the second is the filter on that table expression.

Give this a try:

True Flag Count =
VAR _trueCount =
    CALCULATE ( COUNTROWS ( 'Table' ), 'Table'[ClosedFlag1] = TRUE )
RETURN
    IF ( ISBLANK ( _trueCount ), 0, _trueCount )

I've used a variable so the CALCULATE function only needs to evaluate once.

I would say use COUNTROWS inside a CALCULATE, in the same way to the code I posted earlier. That way you avoid doing a direct COUNT of the Boolean.

Also, instead of multiplying the measure by 100 to get a decimal, you can format the measure directly to be a Percentage type by going to the Modelling tab.