You'll probably want to calibrate the spreader so you get the right amount applied. You'll need a scale and a measuring tape. The simplest way to do it may be to weigh out some fert (say 10 pounds or so), and spread it out over your driveway at a known distance. Multiply the distance by the width of the spread swath, so you know what area you covered. Then, weigh the amount of fert left in the hopper. Subtract that from the weight of th efert you started with and you'll have the weight of fert you applied to that area. You can then adjust up or down and follow the steps again until you find a setting that delivers what you're looking for.
Don't forget the sweep up the fert off of your driveway, so you don't pollute the environment or waste any fert.
On another note, why use 19-19-19? Are your soil test P levels low?