I figure they go about 1 foot down for every two feet of line out, or thereabouts. It's not precise, but it works well enough. This assumes that there is enough current to make them dive hard, and that you're not running into bottom yet. Once they get close to bottom, they tend to bounce off the turbulence layer at the bottom, and run a half a foot or so above the bottom.

That calculation works pretty well for pink ladies, as well.