Kendal, you're smart to ask the question BEFORE you spray, and not to cast dispersions on anyone, but Dennis is correct, you have to work out the math so that both the mix and the application work out for the recommended COVERAGE.
I tried the "spray with water on the driveway" approach, and I mixed conservatively (versus the recommendation I had received) and went with what looked good to me. But when I was all through I had consumed twice as much chemical as recommended for a yard my size. Problem was I had put it on WAY too heavy.
I concluded that what I judged good coverage on asphalt was in fact a severe over application. (Most of my grass is still dead, the weeds are coming back, but the dandelions are gone [except one].) The silly little sprayer is more powerful than all my equipment, even the mower.
I also notice that with my 2-nozzle sprayer, the weeds at the ends of the spray pattern are especially quick to come back. There is method to Denny's madness, by using more heads, and getting closer to the ground, he is able to achieve more even coverage.
I mean to try spraying plain water over my whole yard until I'm sure that I have my speed right. I also picked up some dye to put in the tank so that I know where I have sprayed so I don't miss spots. (My yard looked like it had a bad hair cut a month after I sprayed.)
I'm committed to a three year plan. This was the first year, learned a lot, probably won't get it all right next year either, but it sounds like Denny has invested the time to get his equipment and his application rate down to the science required to be successful.
Good work, Denny, and thanks for sharing.
Edit: The crab grass seems to be really taking off, though.