I remember seeing something a while ago but don't remember where. There are so many variables that it's really just easier to figure it out yourself. I longline a 1/16oz roadrunner jig tipped with a minnow on 8lb test line. I make a strong cast and troll around .8-1.0 mph. This puts my jig down around 10'-12'. It's easy to figure this out when you go over a shallow point and can see that your jig is touching bottom.