Proper tank size for well and lake intake

[MHM]

I've got a house where both the well pump and lake pump cycle on and off a lot. I think the tanks are undersized. The house has a well approximately 100' deep. I don't know the submersible pump size. The tank is a 20 gal pressurized well tank. It takes 19 seconds for the pump to cycle off once it comes on with no fixtures drawing water. The house has 5 toilets, 5 showers, 9 sinks, 3 spigots, dishwasher and a washer for a total of 24 fixtures. It's a tankless hotwater heater. What size should the well tank be? I've found one website that says to multiply the number of fixtures by 3 to get the minimum tank size. In my case, that would be 24 x 3 or the next size up from 72 gal. Other websites call for the pump size to calculate the tank size but I don't know that. What is the proper tank size?
I also have a a lake intake with a 1 HP jet pump that feeds 2 spigots to runs 1 to 4 sprinklers. The lake to pump height is less then 10'. It currently only has a 3 gal tank. Not surprising the pump only cuts off for a very short time before it comes back on. What size pressurized tank should it have?

 

[Valveman]

A 20 gallon tank only holds 5 gallons of water. So if your pump fills the tank in 15 seconds, you have a 20 GPM pump. With a 20 GPM pump you would need a minimum 80 gallon size pressure tank, and if you want the pump to last you would use two 80 gallon pressure tanks to reduce the cycling. However, that was the way we did things 25 years ago. Today there is a much better way to keep the pump from cycling, and you don't need a larger tank on either system. Just add a Cycle Stop Valve model CSV1A after each pump, the cycling will go away, you won't need another pressure tank, the pressure will be constant and much stronger, and the pumps will last much longer. See this video.