Trailing 12 or Rolling 12 month sum

you don't need to use monthname, that is only for average, but this formula you need to translate

Sales12M := CALCULATE (
    [Sales],
    DATESBETWEEN (
        Calendar[FullDate],
        NEXTDAY ( SAMEPERIODLASTYEAR ( LASTDATE ( Calendar[FullDate] ) ) ),
        LASTDATE ( Calendar[FullDate] )
    )
)

So sales in this case will be amount and full date will be replaced with date from your calendar table.

You don;t need to worry about person, once you add this on a visual with person, it will automatically group it by person.