adding data value with start and end dates for all dates

I am a newbie so please be gentle :

assume the following sample data table

using DAX measure

DEFINE 
MEASURE 'Sample Data'[a2] = VAR StartDate = MIN('Sample Data'[Start Date])
VAR EndDate = MIN('Sample Data'[End Date])
VAR dat=
    ADDCOLUMNS(
        Calendar, "Data",
        IF(
            AND(
                'Calendar'[Date] >= StartDate,
                'Calendar'[Date] < EndDate
            ),
            CALCULATE(
                SUM('Sample Data'[Value]),
                FILTER(
                    'Sample Data',
                    'Calendar'[Date] >= 'Sample Data'[Start Date] &&
                    'Calendar'[Date] < 'Sample Data'[End Date]
                )
            ),
            0
        )
    )

RETURN
DIVIDE(SUMX(dat, [Data]),COUNTX(dat, [Data]))

My output:

Issue 1:  I would like  COUNTX to exclude nil(0) values so I can calculate portions of years

Issue 2:  The Grand Totals for columns are inconsistent and incorrect (see 2019, 2023);  

Issue 3: Is this "correct use" of variables; I sort of recall I may have broken some rules doing this  - bad or just VERY bad!

Notes:

1/I expect the Grand Total for rows is OK assuming I(we) fix the Count problem in issue 1. But fix 1 thing at a time

2/Apologies to using "value" as column name, I know - bad!