You shouldnt let it go lower than 5% as the gauge may not be accurate. You dont want to run out as that would require the gas company to do a leak test before they refill and that will cost you extra.

i have a 500 gal propane tank that is 47% full i know its not suppost to get no less than 25% full. anyways my question is do i have to fill the tank up one last time or not? since its getting warmer out we wont be using much? i just recently moved into this house with propane so im still new to it.