Recently, a client who sews called and asked me if I could hem her curtains. She had made the curtains herself. There were polyester marquisette. The selvage had shrunk about six inches, and the bottom of the hem was at least an inch off from one place to another.
She was almost in tears, frustrated. She had tried to hem them three times. And they still were crooked.
So, I went to her house, and I put a row of pins 1/2 inch above the floor. Where they were too short, I released the hem. (If you do this, use safety pins so the pins won’t come out.)
When I returned to my studio, I cut off the shrunken selvages and serged a rolled hem on the sides.
Then, I clipped the curtains on a skirt hanger and hung them from the ceiling for at least an hour. (I have ceiling hooks in my studio.) The row of pins was still an even distance from the floor. So, I compared the sides of the curtains to the bottom and pinned them, too.
Conclusion: When hemming curtains and drapes, always hang them to determine the correct length. Never measure floor length curtains and drapes when they are horizontal (on the floor or table).