This is the fun part...Remember your geometry? Maybe a little trig? You form an imaginary right triangle with the line, the bait, back horizontally to the boat and then up to to the surface.
I try to run the divers at a 45-degree angle, although it depends more upon how well the spinner/bait/etc. is spinning.
At 45-degrees, the sin is about .7 of the hypoteneuse...In other words, .7 of the amount of line you let out. Let out 20 feet at 45 degrees and you're fishing 14 deep.
Now, if you have a smaller diver or troll faster, you may get a 30-degree angle...The sin of 30 degrees is .5 (on the money)...let out 20 feet and you're 10 deep.
Not only seems simple enough, it typically gives me plenty to do between bites on a slow day...
(Used to just rely on geometry's Pythagoreum theorem...the sum of the squares of the two sides of a right triangle is equal to the square of the hypoteneuse...got kind of complicated until the trigonometric functions hit me
....Who knew that someday I would actually use that stuff?)