I always steep my grains first and that water, with top off, becomes the brew water for my extract and hop additions. I suppose your method could be a time saver, but I do a partial boil. I start with 3 gallons, total after topping off the steeping grain water, and add half my extract up front to create a wort that is similar in composition to a five to six gallon boil so that the hop utilization is correct then add the last of my extract in the last couple of minutes. Trying to add my steeping water in the middle of that would throw my "calculated" method all out of whack!!!