It's not an educated guess, it's a simple calibration procedure. You don't set the flow rate with the pump, you set it with the nozzles. They put out X gpm at y psi (exact numbers depend on exact nozzle. They are fixed, not adjustable) You aim for, say, 10 gpa, pick your nozzles and speed to achieve that. Then you calibrate to fine tune it. Not as big a deal when you are spraying 3 acres to overspray by 10%, so you can maybe leave this step out, but we used to spray 1000 acres. An extra 10% cost is bad there. And worse, 10% too little could lead to needing to spray again...
Here's a link with calibration procedure:
Boom Sprayer Calibration, AEX-520-92