Well this is kind of good news. After googling coolant stuff, it sent me back to nasioc (surprise surprise) right to unabombers stickies. and according to him:
What about topping off my system? Don't sweat it. Just add water and be done with it as the minor amount of water you add isn't enough to dilute your overall coolant capacity enough to change much. If you are super worried anyway, you could top it off with pre-mixed coolant, but that's a little on the "it puts the lotion in the basket" side. And if you think you need to add .1 ounces of Subaru Cooling System Conditioner when you top off...you might want to get a new tin foil hat.