depends where you are located. if it snows and gets realy cold in your area, then you should leave it at 50/50 mixture plus water wetter.
here in CA, it doenst snow or freez, so I'm going to go with 30/70 plus water wetter...water absorbs heat better than coolant. but you need coolant to prevent rust and water from boiling.
BTW, make sure use distilled water....not tap water please.