From what your describing it sounds like a wx-202 tank. The important thing and I think "coalkirk" said it , is to let the pump run alittle. The hardest thing on a pump is alot of starting and stopping. The pump get hot starting and stopping all the time. Plus start winding will burn out faster. Water being drawn past the motor when the pump is running is what cools it. Most pump manufactures recomend at least about 1 1/2 minutes of run time. My chart shows that at pressure of 20-40 you will get 8 gallons of water from tank before pump kicks on. If your pump is pushing about 5gpm your at about 1 1/2 minutes of run time. If your pump is pushing 7 gpm your short of normal run time. Error on the bigger side since you don't know pump size, depth, ect... I'd install the next size tank up. In amtrol tanks that would be a wx-203. Amtrol is pricy, Ferguson plumbing supply carrys their line of well tanks(proflo) and they are made for them by, Amtrol. The proflo tanks are better priced but your still getting amtrol quality. I heard "State" mentioned, thats a good line also.I would install a low water cut-off pressure switch while your changing the tank since you said you can run it dry. Hope all this helps.