I never just add coolant as I always bleed the system whenever I top it off. Therefore I top off to the top of the fill neck and it self-levels after driving it. I wouldn't recommend removing the bleed screw or radiator cap when the engine is at operating temp. I had a radiator cap blow of and cover me with boiling water at a car meet one time. I think the same thing happened to wolferj. The pressure caused the coolant to rush out like a geyser shooting about 5 or 6 feet in the air and it practically emptied my radiator. This was after the car sat for about an hour and I thought it would be safe to top it off(had to bleed it daily due to a cracked radiator). I'm pretty sure the BMW antifreeze is pre mixed 50/50. Too much water can allow it to freeze and I think it also lowers the boiling point so be careful not to over dilute it
