Paying Zakah from bank account mixing salaries, deposits, expense funds

In EU money is deposited in the bank and the salaries usually go to the bank straight away which means savings and salaries are mixed. This means it is very hard to keep track of the period for each amount, since in practice we use the same account to withdraw for daily expenses. So every Ramadan, I see how much in the bank and I pay 2.5% for Zakah. This means I am paying more than what I should. For me there is no problem. When people question me on this issue, is it right say to them to follow my method after I explain it to them?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

1. As a general rule depositing money in an usury-taking bank is not permissible. If you do not have any Islamic bank and fear money will be destroyed or stolen, only in this case you may deposit your money in usury-taking banks. If those banks provide you interest on your money you have no right to possess that interest; so you should spend it on Muslims' public welfare.
2. If one possesses an amount that reaches Nisaab (minimum amount of property liable to Zakah that is 85 gram of pure gold or its equivalent) and, after having this amount, he also receives money from other sources, for instance he saves from his monthly salary, or receives presentations, gifts or rents on real estates, etc. then he can follow two ways to discharge his Zakah.
First, if one does not want to spend more than his obligatory duty then he might maintain a calculation of the growing money and pay the Zakah of every amount on its time, i.e. after one lunar year has passed after possessing it.
Second, if a person is used to paying Zakah on a certain month (e.g. each Ramadan), then it is better that he pays Zakah on the whole amount regardless of when it exceeded the Nisaab. This latter practice is preferable if the person wants to be generous to needy persons, he is then prepared to spend the additional money to benefit Muslims, and he receives more reward for doing so. Know that paying Zakah before completing a year is permissible; it is considered as early paid Zakah. Therefore, you may guide people to the latter way of paying Zakah by explaining them its merits. Allah knows best.