If there is a straight line between the two other pulleys that's around the AC's clutch, then all you need to do is route a string (around all of the pulleys minus the AC) and find how many inches of belt you need (position the tensioner to its extreme - most tension). Most belt manufacturers embed the belt length in the part number. So for example, if you have an Advanced Auto near you, they will probably sell Dayco Poly-Rib (or Poly-Cog) and the syntax for this belt is 5060[x..] where the [x..] is the length in inches with the decimal removed. So if you need a belt that is 73.5 inches long, the part number would be 5060735, and for a belt that is 80 inches long, then the part number would be 5060800. Once you have that number, buy three or four belts with of lengths in 0.5 inch intervals with the measured length somewhere in the middle - this will minimize trips to the parts store and the ones that are too long/short can be return afterwards.