I just estimate the depth by the angle the line takes into the water at your desired speed. For instance, if you are fishing 10 foot forward from the back of boat and your line enters the water at the back when you hold your tip 4 foot above the water, then a 30 ft. cast will put you around 8' to 12' down depending on how low you move your tip. Close enough, as long as you can hold speed.
