Is my method of paying Zakat on the lowest amount in the account that has completed a Hawl (lunar year) correct? And how can I clear my liability for past years if it is incorrect? And what is the correct method for paying Zakat annually?

What you are doing is correct: the lesser amount throughout the year is subject to zakat (definition) when the hawl (one lunar year) of the original capital is completed. However, if the wealth increases after that, a new hawl begins for this increase from the time it was acquired, and its zakat is due when its hawl is completed. It is not permissible to delay its zakat until the hawl of the original capital. The easiest way to pay zakat is to look at all your possessions at the end of the hawl and pay a quarter of a tenth (2.5%). If the hawl has passed on it, then so be it. If the hawl has not passed on it, then you would have advanced its zakat, and there is no harm in that.