There is no rule of thumb for line length to depth, there is however a great app called Precision Trolling Data, which has dive data giving the line length to depth values for hundreds of lures. For example, the book says to run a #4 Berkley Flicker Shad 4 feet deep, let out 26 feet of 10# Berkley Trilene XT. To get the same lure down to 6 feet, let out 94 feet of line. This app (available for IOS and Droid) is a very valuable resource for anyone that trolls.
Good Luck Walleye Fishing,