Help on Creating a percentage measure from a current value against a previous value

ID Count = DISTINCTCOUNT( Movements[ID] )

% Moved = 
IF( ISINSCOPE( Movements[CurrentDateText] ),
    var CurrentPeriod = SELECTEDVALUE( Movements[CurrentDateText] )
    var PreviousPeriod =
        TOPN(1,
            FILTER(
                ALLSELECTED( Movements[CurrentDateText] ),
                Movements[CurrentDateText] < CurrentPeriod
            ),
            Movements[CurrentDateText],
            DESC
        )
    var CountPreviously =
        CALCULATE(
            [ID Count],
            PreviousPeriod
        )
    var CountNow = [ID Count]
    return
        DIVIDE( CountNow, CountPreviously )
)

I called the table Movements, obviously...