TERRY - You "Calibrate" your sprayer the same way I do! Spray water on the concrete and watch it evaporate!
I spray with a GD CC since they hold speed so much better, and using the 12V pump and the nozzles I get the right ground speed at 1800 RPM in 1st gear. My boom covers 80 inches. I've had my sprayer for 15+ yrs and used everything from Weed-B-Gone, Weed-Stop, and this year I used Acme Amine 400 2,4-D, it all seems to work good for broad leaf weeds. I also use the sprayer with the hand wand to spray Round-Up around trees & bushes sometimes.
I made the BIG sprayer a couple yrs ago to spray a wettable powder called DIMENSION for crab grass control, has to be applied early spring just before the crabgrass germinates, I think I was a few days late getting it down but it did really help reduce the amount of crabgrass I had, it's just REAL expensive. I needed the higher capacity (GPM) roller pump to get more recirculation to keep the powder in suspension, it runs off the frt PTO, and the nozzles are sized to allow running about the same 1800 RPM in 1st gear.
The little sprayer is an ATV sprayer, 14 gal tank & mounted 12V diaphram pump which only makes about 1-1/2 GPM @ 60 PSI.
I've always sprayed a day or two after mowing, but I suspect you could spray behind the mower just as well.
I also mow about 2-3 rounds then mow back & forth with 180 degree turns on each end. My yard is almost triangular and has about 80 trees & bushes, so even at over 2 acres, I very seldom get to make a pass without having to trim around a tree/bush.