dynamic topN selection

you will need a table that actually has "Others" in it, something like this (countries in my DAX code)
countries

that table needs to have a single direction 1:many join with Sheet1 table

then you create measure Rank

Rank = 
RANKX(ALLSELECTED(countries[countries]),CALCULATE(SUM(Sheet1[value])))

the TopN slicer measure (can be done with WhatIf parameter)

TopN Value = SELECTEDVALUE('TopN'[TopN], BLANK())

and then the measure with value

Measure = 
VAR __TopN =
    TOPN ( [TopN Value], ALL ( Sheet1 ), CALCULATE ( SUM ( Sheet1[value] ) ), DESC )
RETURN
    IF (
        [Rank] <= [TopN Value],
        SUM ( Sheet1[value] ),
        IF (
            SELECTEDVALUE ( countries[countries] ) = "Others",
            CALCULATE ( SUM ( Sheet1[value] ), ALL ( countries[countries] ) )
                - SUMX ( __TopN, [value] ),
            BLANK ()
        )
    )