I've found that my Abu Garcia 6500's offer enough line capacity to accommodate lead core line. Lead core line itself is metered in yards by exhibiting slight color changes in the braiding. The markings allow you to keep track of how much line has left your reel. In short, the more color changes you see spooling off the reel, the deeper your crankbait is running. Lead core line effectively forces a shallow running Rapala to dive deep.
Line weights are another tool for achieving maximum crankbait depth.
The new Ultra System from Bullet Weight not only pulls crankbaits downward, they are also designed to make changing weights simple, without re-tying. The best way to determine when maximum depth has been reached is to watch your rod tip. Continue adjusting line weight, and or the amount of line released behind the boat, until your lure begins hammering into the bottom. Once contact is made, crank your reel a few times and your lure should be humming right above the bottom. An Offshore Snap Weight can be placed in front of a moving crankbait to control lure depth.