From where are you using this method?
Can you adjust the whereclause to get only the one you need?
If not, usually webService method are called within a script/program or something similar. The method returns a list of string, you can convert it to an array, and access the last index.
Ex : array[array.length-1] depending on the language beeing used.
If your language support XML parsing or XML Node naviguating, you can read it directly with a similar approach.