Sorry to say but that's just not correct. You want to hit your target gravities to get your beer to come out as expected, and to do so, you need to look at how much water you are boiling off over a period of time. There are a lot of other factors with regard to this as well. Your altitude will cause you to have differing levels of boil off as well. Ever seen a recipe with high altitude variations for preparation? It's the same principle. If you want five gallons of beer, you need to know about how much water you will loose per hour at a given boil rate. This goes for both all grain and extract methods. All grain makes it even more important to know about water consuption for reasons not to do with boil off.