Set the tax to 1 each day then. If someone can't even pay 1 a day, they really don't have anything to complain about. They'd just have to go out, get one single dirt/cobble, and then sell it.
That's honestly not that hard to do, even for a new player. Just stress this fact to them and make sure they understand it. Then the duty's not on you to ensure they keep enough money in the bank, as they then understand and know what'll happen if they don't even keep 1 on them.