If the amount of rows can change, you could try something like this:
SELECT TOP((SELECT COUNT(*) FROM MyTable) / 2)
*
FROM MyTable
ORDER BY ColumnThatDefinesTheOrderForPercentage DESC
Or simply use
PERCENT
and order the data in descending order, like:
SELECT TOP(50) PERCENT
*
FROM MyTable
ORDER BY ColumnThatDefinesTheOrderForPercentage DESC