The best way to do this is with a peice of string or wire. Route the string around the pulleys the way you would want it to run. Being careful not to route it so as to spin the water pump backwards. Then take the measurement of the string. Whatever the inches equals these will be your first numbers in you belt part number. Example.(64 1/2 inches= 645.....64 inches=640...63 1/2 inches=635....and so on and so on.....) Then the last number/ letter will be "K6". (Meaning 6 ribs on the belts). This is how serpentine belts are numbered. So if you come up with 61 1/2 inches the belt you need would be "615K6"
Hope this helps.
Another tip is if the pulley has grooves for ribs then the rib side of the belt should be on it and if the pulley has no grooves then the flat side of the belt should be on it in order for proper rotation of pullies.