I originally asked this question which I am splitting up.
Fluctuating Net worth: Some months, one saves more and some months one spends more due to various reasons. How does one establish the correct "wealth" to pay zakat on? Since according to definition, it has to sit idle for 1 year before we pay zakat on it. The local imam I talked to suggested to take an average of the two net worths and pay zakat on that.