Don't forget to allow for the transformer regulation factor, annd also mains supply ('line') variation, which will affect the peak voltage seen by the buck converter. e.g. if the transformer has 15% regulation and is at low load current, its output voltage will be up to 15% higher than at full load current. This compounds with line variation so at a high line limit of +10%, you could get as much as 43V peak. Its going to be tougher than I'd like to find an off-the-shelf buck module that can handle that with a wide enough input voltage range.
Low line obviously affects the peak voltage the reservoir capacitor charges to, which means it needs to be larger to store enough energy to ride through the zero crossings. Sim it at Vin 10% low, with worst case minimum capacitance (typ. -20% for a good grade of capacitor, and if its wet electrolytic, allow for further loss of capacitance due to ageing), and be prepared for an unpleasant surprise. To satisfy the ripple current requirement in limited volume, it may be easier to split up the reservoir capacitance between two (or more) capacitors.