A spring scale attached to the idler and a stationary point on the deck with zero as the full slack, zero tension point and then tighten adjustment to 30lb.The belt in picture is a $30 aramid fiber from Amazon made in India, the OEM belts are $70 for the fabricated deck. The owners manual states belt needs 30 ft lbs tension but I am not sure how to measure that?
My guess is it's not hooked to the belt but to the tensioner idler pulley. Otherwise if there is a specification for the belt it would be amount of deflection of the belt but you can't do that because the tensioner would continually be taking up slack to the limit of the spring. That sort of measurement is for things like a belt between two or more stationary points.Where would the fish scale be hooked to the belt?