With extract, as long as it all makes it to the fermenter, you will get the correct OG if you get the right volume. There is little to no variation in the gravity points that extract will add, and sugars do not boil away. If your kit says it makes 5 gallons of 1.045, then thats what you have. Only exceptions to this rule would be not putting all of the extract in, or a boil-over. As far as evaporation I usually plan on losing 10% or so. In regards to the grocery store water, if the water from your tap tastes good when you drink it, the it should be fine in your beer, too.