Not sure where you are but in Rural districts the local tyre stores carry A, B & C section belts as a stock item. Best way to work out the length is to tie a length of sash cord around all the pulleys and tie it or mar it to length. Use a small piece of sheet metal and crimp it around the rope so that it is at your m,arks. Take this to the tyre store and they have a belt gauge that measures the length and then they either get one off the hook up on the rafters or order it in for you.
I used to do this for the farmers who came in with all sorts of variants of measurements. To make sure the size belt A, B, or C measure the width of the pulley where the belt sits, this will give them the size belt you need.