I would expect the tolerance to be in percentage, or linearly, in proportion to the length of the belt. If a 3" variation in length for the same part number (3.14% in this case; a 50" belt would be 6%) is the best the manufacturer can hold, they need to get out of the v-belt business. Maybe they should get into weather forecasting. For more info on measuring v-belts, see: https://www.wikihow.com/ Measure-a-Pulley-Belt-Size.