Remember that Snap Weight diving depths are affected by 3 factors:
1. The weight of the Snap Weight
2. The amount of Dropper Line you let out (line length from board to weight)
3. Your trolling speed
I like to just change one variable in the equation. I pick a Snap Weight that will be big enough to get to whatever depth I want – probably the two most common are 1 ouncers in 20 feet or less, 2 ouncers in 40 feet or less.
So, once you have decided on your speed and the weight of the Snap Weights you are going to use next you have to determine your dropper lengths. Like you mentioned, you can use the Precision Trolling book if you use the 50/50 method (50 ft leader/50 ft dropper) and your speed lines up with one of their graphs.
If you want to change things up, then use the bottom to give you line lengths. I simply drop my Snap Weight down to the bottom. Let it swing up and then drop it to the bottom again. Swing up and drop back again. Now your weight should be running very close to the bottom.
Look at your line counter and you will know how much line to let out to get to a certain depth with the Snap Weight you are using and the speed you are trolling.
Now to cover other depths, do some simple math. If you were in 30 feet of water and it took a 50 foot dropper to hit bottom, then 25 feet of dropper will get you 15 feet of depth. A 10 foot dropper will get you 6 feet and so on.
This will get you in the ball park – remember to spread out your Snap Weights at or above where you are marking fish so you can figure out the Eating Zone.