This is the 1st attempt and measurement on my car it is now only 1/4" bellow the cross member. Don't go any lower than where you determine the most of you drag to be coming from. So take a look, determine how much is if any thing is below your factory dam and adjust accordingly.
You may have noticed that some of the factory dams are higher in the middle, I believe this is two fold one to keep the frontal area as small a possible and still redirecting the air flow around lower hanging components.