From time to time I get emails asking me how to do certain things, sewing-wise. I wish I had time to write tuts on all of the questions that I receive. The question I was emailed today is, how do you mark a floor length gown for hemming? Well, there are lots of types of hems. There's no way to cover all of the bases, but from her description, I'm assuming that this person is asking about a hanky or baby hem on a light chiffon or georgette gown. The bulk of the fabric really matters when estimating how much fabric to leave for your hem allowance. The bulkier your fabric, the more length you need to leave.
While I illustrate how to mark, I'm going to go ahead and give a few cutting and hemming pointers as well.
Lets say the client wants a 1/4 or 1/8 inch rolled hem. This is straight-stitch, not serged. I would take that number and multiply by 3 to get 3/4 inch. I would have the client stand on a 1 inch magazine in the shoes that she plans to wear with the gown. (The magazine will compress slightly when she stands on it.) I would mark to the floor all the way around. The more marks you have the more accurate. Have her look straight forward, she's not allowed to look down. That picks up the back of the dress. I use a sliver of bar soap to mark. Make sure that you do not stretch the dress as you mark. Mark it right where it naturally hits the floor without being pulled.
Cut on the long side of the marks, (always err long on every decision, you can always shorten a bit if needed later.) When you approach a vertical seam, swoop down a little bit. The extra bulkiness will gobble a little of your length. (see my blue marks in the 3rd pic)
As you sew around the hem, snip those seam allowances on the vertical seams. That will make them less bulky.
One huge tip to note when sewing the baby hem: It's not just about the right hand holding a tight little roll. The left hand needs to pull to the left while the right index finger pushes down and pulls slightly to the right. (see gray arrows) This pulling from both directions keeps the feed dogs from causing your roll to pull extra fabric into it. -That would cause the roll to be bigger and less tight, (yuck!)
Hope this helps! :)