A real easy way to get you close to the right starting length is to simply measure it with a flexible (fabric) tape measure. Put the belt that's too long on the machine, make sure it fits well on the two clutches by just holding it by hand or putting a couple zip ties around it (between the clutches, so it's squeezing the belt tight on the two clutches. At this point, you can take the tape measure and figure out how long the actual belt needs to be by essentially "making" the belt with the tape measure. I loop it around the bottom of the clutches (over the top of the zip-tied, too-loose belt) and then pull it tight on the top and get the measurement. Make sure the engine is moved the entire way forward in its adjustment before you do it. You can add a 1/4' to a 1/2" in the measurement just to be safe. I do this each time I put a custom machine together, since I'm not smart enough to use the center-to-center/pulley diameter method.
Here are links to a couple pictures showing excerpts from the Dayco belt catalog with dimensional data and part numbers. Make sure you get the right top width, too. There's a .pdf of this in the brochures section, but I can never download it correctly.