It could be that the pivot pin is used because it is easy to calculate, only one cylinder is involved so some wiz bang engineer can figure how much force is available working at whatever fulcrum points, leverage, etc.. The distance to the center of the bucket works in an arch, you would have a different lift figure with the bucket flat or with it curled back or in the dump position. 30% may be a bit much or it might be right on the money. I have a couple tons of tractor weights and could do a test, but I expect the rated lift considers a reserve or margin of error so I may be able to lift more than the recommended amount.