Hey Rip,
If we have any math teachers out there I'm sure they will use this for a test question! The problem is there are too many variables to answer your question. First, you're trolling speed would have to be constant. Second, the amount of line pulled out would have to be the exact same each time. If you know those two factors and keep them the same, then you take the angle that the line makes when it breaks the plan of the water. If you know the angle, and you know the amount of line out, you can then determine the depth. Because if you know one angle, and make the second angle 90 degrees, you then will know the third angle. Once you know two angles and one side you can figure out the other sides (ie the depth). But that's as far as I can take you since I haven't figured this out since high school. You'll need a high school trig book.
But the problem is to know how to vary the depth every time you go out you will have to measure the angle of the line as it enters the water. Pretty hard to do on a moving boat. Even with a downrigger we are using rough estimates of depth since the downrigger line will slant back. So the real answer is: keep your trolling speed constant, adjust the amount of line out until you hit fish, then keep track of the number of pulls you used to get to the fish and use the same pulls the next time. (Which is basically how you use a pink lady or deep six driving plane anyway). Man - this fishing stuff is way too complicated!
I use a rough rule of thumb - you're depth is about 2/3s of the line out if the line angle is around 45 degrees. So 60 feet out equals about 40 feet deep.