I think you're right, Astroboy. I tried to find a "close enough" value that could be used as a general guide, but I do realize there is room for error, and each case is a case.I've been reading this thread with some interest and thought I'd chip in. To calculate the length of any belt rather than working backwards from a stock setup and known belt length and adjusting according to percentages, which seems a bit hit a s miss. What you need to know is the distance between the centres of all pulleys (including tensioners in their optimum position) and their relevant positions. The diameters of the working surfaces of the pulleys AND more importantly the routing of the belt. Putting it simply, if you need an exact answer every time you need a CAD programme
Edit. Nearly forgot. If the belt runs on its inner and outer surface (against a tensioner for example) you also need to know the thickness of the belt which will vary according to manufacturer. You also need to know if a manufacturer measures its belts on the inner or outer surface. Best of luck!
Have you tried the file on this post;HI.. Does anyone know the belt size for a ALTA 17% pulley with a ALTA+2% crank pulley, 6PK1353? 6PK1370? OR another SIZE?