Можете да COUNT
всички записи и след това изчислете %
имате нужда от това:
$query = "SELECT COUNT(*) FROM `Flight_Data` WHERE DepDateTimeUTC LIKE '%1/1/14%' ";
$result = mysqli_query($connection,$query);
$row = mysqli_fetch_row($result));
$total = $row[0];
$percent = intval($total * 0.40);
$query = "SELECT * FROM `Flight_Data` WHERE DepDateTimeUTC LIKE '%1/1/14%' LIMIT ". $percent;
//execute your query....