You have to remember that these threads are often read by others trying to learn about equipment choices, and it's for the now and future benefit of a wider audience that I stress some of these points, because every day, someone looks at a cast-iron "lawn sprinkler pump" and thinks that's just the item needed for a sprinkler system installation, not knowing the designation is either outdated or downright deceptive. Today's common rotor sprinklers need much more pressure than the simple brass spray heads of yesteryear, and a system greatly benefits from having a water source with more pressure than a centrifugal pump can manage.
So trust me, no one's beating up on you. It just happens that you walked into a teachable moment. Smile!
Meanwhile, your pump is going to be performing as its curve describes, and to make the combination of pressure switch and pressure tank work for you, you will be fine-tuning the cut-off pressure adjustment, so that you get a full minute of pump operation every time it starts, on the way to the tank pressure going back up to the cutoff point. This is where we get back to the script that was being read to you, and what the larger tanks make possible.
The larger the tank, the lower your cutoff pressure can be, so long as you are having the same cycle interval. You want an interval of at least one minute as a safe minimum, and a pro setting up a deep-well submersible can be thinking in terms of two or three minutes between pump starts. So this is where you have a tough choice to make, because this is an application where bigger is better. The only downside to a bigger tank is that it costs more money. The downside to a tank that is too small, is that it won't be able to give you a full minute of runtime for your pump.
The pressure switch itself is a fairly simple mechanical item (I recommend a Square D Pumptrol FSG2) and they have their limitations, in that if you set a cutoff point of 43 psi, it will most of the time cut off at 43 psi and at other times may cut off one or two psi higher or lower. This detail is important because the absolute maximum cutoff pressure you might set for your pump is 47 psi, the pressure that it would be deadheading at. (see the chart)
So the bigger the tank you buy, the more wiggle room you give to your pressure switch adjusting. And you need that wiggle room, because the difference in output pressure with you running a hose, and the pump simply deadheading is going to be a very small difference. What works in your favor, is that a cycling pump with you running a hose will take a bit longer to reach cutoff pressure (but not so much longer that it makes up for an undersized tank)
Here's a simpler calculator to play around with, but no online calculator will take into effect that your pump flow rate is going to start out big and end up small. The selection of a tank is going to come down to your budget and willingness to invest in a reliable level of pump protection. See what tanks are available in your area, and compare prices and capacities and reviews of quality. I think you should start the search with a 32-36 gpm precharged tank.