Dude, never add straight water to a sump. It will separate. This is first hand info from The boys at Blaser Swisslube, and Ometa. They all say the same thing. DO NOT ADD Straight water.
Make some 1% and add that. Generally if you charge your machine to say 10-12% the coolant you chould be adding later should be in the well less than that, depending on the amounts you are required to top up. This will only come from experimentation!
"It's only funny until some one get's hurt, and then it's just hilarious!!" Mike Patton - Faith No More Ricochet