Determine how to sub-divide recalibration analysis based on read length.

def _positions_to_examine(db_file):
    """Determine how to sub-divide recalibration analysis based on read length.
    """
    conn = sqlite3.connect(db_file)
    cursor = conn.cursor()
    cursor.execute("""SELECT MAX(position) FROM data""")
    position = cursor.fetchone()[0]
    if position is not None:
        position = int(position)
    cursor.close()
    split_at = 50
    if position is None:
        return []
    elif position < split_at:
        return [("<= %s" % position, "lt%s" % position)]
    else:
        return [("< %s" % split_at, "lt%s" % split_at),
                (">= %s" % split_at, "gt%s" % split_at)]