This is very easy to say, but can be difficult to do. Proper depth not only refers to what depth your is at, but also what depth of water you are fishing over.
Speed can vary greatly depending on wind speed and direction.
If you keep records, you will probably find that on a given day you caught most of your fish going either with the wind or against the wind, but not both directions. Trolling is not a haphazard exercise of randomly driving around the lake, but a set of exercises aimed at finding the fish and getting them to bite. Then repeating the conditions that worked, over and over. More about proper depth. Even if you have a depth sounder, you should have a topographical map of Sheridan Lake.