Five things affect how deep a weight will take a crank bait.
1.) The amount of line you let out before attaching the weight. Remember that the crankbait will dive below the weight.
2.) The amount of weight you attach. I like to use Off Shore Tackle Snap Weights – for trolling crank baits I mainly use the 3 ounce weight. This is the biggest weight you can effectively pull below an Off Shore Tackle OR-12 Side Planer board.
3.) The speed you troll – slower the speed the deeper the weight will go – pick up the speed and the system won’t get the lure as deep.
4.) The type of line. Thin line like 10-4 Fireline (as compared to a 10 pound mono) will have less resistance in the water. The bait will dive deeper and the weight will sink lower.
5.) The amount of dropper line. This is the amount of line from the weight up to your rod tip (or board if you are using one). Obviously the more line you put out the deeper the system will carry your bait.
So use a 3 ounce weight, at 1.5 mph, with say 10XT as a line and a 50 foot leader I would guess you would have to let out a 50 foot “dropper” to hit 30 feet – probably 75 or so to hit 40.
If you want to be real precise, take the set up to a sandy shoreline. Start deep (say 40 feet) and see if it hits bottom, if not move 5 feet shallower and try again and gradually troll from deep to shallow until it starts to bump – then you know for sure.