To start with you should read the articles on the home page of Crappie.com by Special K, Crappie Fishing 101, I have found them to be very good. They explained the migration very well I thought.
On Truman the fish we have been targeting are moving into the shallows of 12 to 4' FOW right now starting to stage for the spawn. They are very scatted right now, hence the riggin has been good as you can target a lot of water. I was told once that the crappie this time of year want warm water, where is the warmest? Shallow water is the warmest, so I always check some shallow water just to see if they are there. Many times if the Sun is shinning you will find them 4' down to keep the sun from their eyes, on cloudy days it is common for us to catch them 2' down. You tell some people you caught limits of dandy dish 2 to 4' deep in February they will say no way! However on Truman as it has been said a hundred times if you have to pick a depth, 10' will be it, works year round.