If you can not find the information on the net take a string and run it around the belt area, mark the length and measure it. Make sure you have the right tension on the string so it is tight but not to tight. Your tiller should have an engage tines and an unengaged lever, the difference between engaged length and unengaged length is about an inch on most things. Your local hardware store should have one.
The things I do are an evolution and I am always learning. My way is not the only way of doing things, and I may and will change the way I do things as I learn better ways. So any advice that I give is in that spirit.