How is zakat paid on a monthly salary, part of which is saved and part spent, without precise accounting of the amounts deposited? And is the calculation method "amount × 2.5 then divided by 100" correct?

If wealth reaches the nisab (minimum threshold) and a full year (hawl) passes, its zakat (2.5%) becomes obligatory. Wealth acquired during the year should be subject to zakat upon the completion of its own independent year, or it may be given along with the zakat of the original wealth as an advance payment, if calculating separately proves difficult. This is the view of the majority of scholars. However, if the acquired wealth is generated from the original capital, such as trade profits, then its zakat is given with the zakat of the original capital. For ease, a specific day in the Hijri year can be designated for paying zakat on all wealth present on that day. In this case, some wealth would have completed its year before zakat is paid, while for other wealth, its zakat would be paid in advance.