they make something called a depth of troll, not sure who makes it or where to get one, but ive heard of them. they look at the angle your entering the water at, and the length of line out, and give a good estimate. the best thing to do is get a depth finder, and try to guess where your at, then when you go by the shallows, youll know how deep you are if you touch or not. I think with 1 oz banana weight we were down just under a foot per pull last weekend. The other thing is, dont go too fast, kokes like the speed around .8 to 1.4 or so, and if your trolling to fast, your gear will not be as deep. just keep trying, youll get it, Brian
Other than everything said already. just pay attention to the fish finder and how deep the fish are. pay attention to how much line you are letting out. make several passes over the fish each pass letting out more line until you get one. then you will know how much line you had to let out to get that deep. don't have a finder. just guess at depth and when you start catching keep going to that depth by counting pulls.
This should be a simple a squared plus b squared = c squared type math if the line is entering the water at a 45 deg angle and you have 100 ft of line out you are probably trolling at a depth of 60 ft or so.