If it were me, I'd install my dado blade in my table saw with 3/8" width. Put my fence at 2-3/8", run a test board through on one side, flip it then run it through on the other side. That will give you a dado that is smaller than 1/2" but then you can gradually move your fence in, run the board through on both sides until you get a 1/2" dado. This method will give you 1/2" dados and you will be certain they will be exactly in the middle of the board.
Do this on a test piece since my math may be off slightly. However, the process will work every time.