I have tensioned the belt to about 5/8" to 3/4" deflection with a 10lbs load at the mid point on the lower run. It's not easy to gauge it accurately without the correct Harley tool, you need 3 hands to do it with a straight edge and spring scales. Essentially, try it a little slack rather than tight but not too much. Give the belt and gears a clean with soapy water and align the rear wheel using the axle adjusters. My car was bad but seems to be OK now, so one or all of those things has sorted it, for now. The sprays and oils are only a temporary fix in my experience.
