Not the best idea to have too much water in the mix. At worst I think it's safe to have a 50/50 mix, depending on how cold it gets where you live. I'm pretty sure you can pick up a kit at a parts store that will let you know the freezing point of your coolant. If you have some coolant just pull some of the mix out with a turkey baster and add some more non-diluted coolant to the mix. Do you have a rough guess at the water to coolant ratio? I don't have access to my Bentley but I know at one point I did have a picture of the allowed mix ratios, I'll see if I can find them.