I have been studying this too and about to change an old glass lined tank with a modern bladder or diaphram. I have learned so far that the bigger the better if: you have the physical space, and can afford it. So while the formula asks for # of people in the home, number of fixtures, and weather gardening and lawn watering, and filling pools or hot tubs, or whirlpool should all be factored in to target the proper size tank. Our goal is simply not to short cycle the pump (meaning constantly cycling on and off which could shorten the life if the pump). However no one can tell me how many times this should happen in a day for an average home, so I hooked up a counter to my current pump and monitored it for over a year. I learned that my pump will cycle up to 37 times a day if my tank is not charged with enough air and If I charge it properly it will only have to run half that time. Note my tank is air over water meaning the air is in contact with the water that is why the air is lost in my system and it needs to be recharged every 6 months or so.
