Okay, I've been here myself. After close to a year, I could not find any data on the subject. But due to lack of information, I don't know what you think you are asking. Percentage of power transmission will be modified by the tension in the pulleys. The belt needs to fit the pulleys. The pulley sizes determine what is effectively the gear ratio, which determines your RPMs. So your torque could be quite a few values.

However, if you are trying to reverse-engineer the belts and just want data, I recommend checking out McMaster-Carr's website or app. You will find some good data there that may apply.