Simple Paging help?

I am trying to setup simple paging - which works great - but if I limit the intial select to say, SELECT TOP 50, *, etc. - The below will not work as the rowNumber is out - do I need to somehow re-seed the row number or...?

DECLARE @startIndex int, 
@pageSize int , 
@totalCount int

SET @startIndex = NULL
SET @pageSize = NULL

DROP TABLE #summary

SELECT *, ROW_NUMBER() OVER(ORDER BY hcpcs DESC) AS 'RowNumber' INTO #summary FROM dbMasterdata.dbo.tblADDB
ORDER BY hcpcs

UPDATE #summary

SET @totalCount = (SELECT COUNT(*) FROM #summary)

PRINT @totalCount

SET @startIndex = @pageSize * (@startIndex - 1)

SET @pageSize = CASE WHEN @pageSize IS NULL THEN @totalCount ELSE @pageSize END
SET @StartIndex = CASE WHEN @StartIndex IS NULL THEN 0 ELSE @StartIndex END

SELECT * FROM #summary
WHERE RowNumber BETWEEN (@StartIndex + 1) AND (@StartIndex + @PageSize)
ORDER BY RowNumber 

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