The thermocline is starting to show up. Generally speaking leadcore sinks 4-5 feet per color. Like others have said it varies depending on a lot of factors. At the speeds you say you are trolling however I would say that your lead is sinking 4-5 feet per color. Right now with the thermocline showing up I would say you want to be fishing 7-10 colors out or set riggers between 32 and 42 feet.
