What is the ruling on giving one-quarter of one-tenth (2.5%) of one's money as an annual zakat, with the intention that anything exceeding the obligatory amount is a Sadaqah (charity)?

What you are doing is sufficient for you and discharges your obligation. It is in accordance with the school of thought of Abu Hanifah, who holds the view that newly acquired wealth (al-mal al-mustafad) is subject to zakat along with the original wealth if a whole year has passed over the original wealth. In contrast, the majority of scholars believe that newly acquired wealth is subject to zakat based on its own separate year. What you are doing, by expediting the zakat payment and setting a specific day for its disbursement at a rate of 2.5%, is a good and permissible act.