Get a rope or cloth measuring tape (such as used with measuring to fit suit or pants) line it up as you would a new serp belt and that will give you the accurate measurement you are looking for...Then take that measurement and cloth measuring tape to the local auto parts store and get your belt...That's what I did....
Oh, take into consideration the tensioner pulley will move when a belt is finally fitted so reduce the final measurement maybe just a wee tiny bit.....