Basically, what toastie has said, is how I do it.

Remember, you can use the grab-and-pull with the mouse to adjust it after the draping occurs. 

Anothe trick to consider, is having the dress train start shorter than the intended length, (in order to ease the initial drap sim), then gradually extend it by degrees to the final length.   That way MD really has to work with the newest length in the calculations.  See the example...it starts as a floor-length gown, and I extended the back portions (and pulled the train around to the front to show it off.)

RosemaryR
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"This...this is magnificent!"
"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."