but long enough to show you're a lady.
I guess the point is a lady would leave a little more to the imagination. Go ahead and tease them with a little cleavage - and I mean on top, not your bottom! - but don't give away the whole show. Would you watch a movie if its trailer told you the whole story? Wouldn't you be more excited to watch a movie if its preview left the ending out? That's why all TV series end in a cliffhanger: to entice you to watch the next episode or season.
Believe me, I am no Louisa May Alcott. I looove wearing short dresses. Especially to give the illusion that your legs don't end! But there is a considerable number of inches between your crotch and your knees. Go ahead and keep the length of your dress further from your knees than your crotch. But please... make sure your dress does NOT end where your panties begin. A little bit of clearance (about 3-4 inches) will make a world of a difference: from shameful & tacky to sexy & tasteful.
And for that sexy illusion you were going for, bring out those platform Pradas! You'll feel on top of the world!