Jenn’s post hits the nail on the head.
Do yourself a favor and buy a copy of the Precision Trolling book, by Holt, Irwin, and Romanack. You’ll be glad you did.
With all the different makes and models, it takes allot of guess work of putting your crankbait in the fish zone. The book has profiles of most of the common cranks and serves as an excellent starting point.
Like Jenn said, the book is written with dive curves for 10 pound mono, but there is a conversion chart in there if you prefer Fireline or different diameter mono. I like using Fireline because I can usually take most cranks down deeper than with mono and find that I don’t need as much line out.
As Jenn mentioned, check the line counter to see how much line you have out to duplicate this factor. I also glance at my depth finder and check out the speed and depth. (I have a paddle wheel on my transom to monitor the speed.)
A line counter can get pretty heavy if you troll all day. It will have a tendancy to flip upside down (must be my carpal tunnel). I find it less of a strain when trolling with the reel facing down as I do like to pump the rod every now and then.
If you prefer to put it in a rod holder, make sure you have a rod holder that will hold your reel securely and the drag set properly, otherwise if you snag something or get a southbound walleye while trolling north you might experience the “slingshot” effect and watch your investment leave the boat rather quickly.
I mention that because I was trolling last fall with my neighbor and he picked up a nice line counter, rod, several colors of lead core and a nice crankbait to boot. All I caught that day was a cold.
Good Luck