But would we have to trade the 3/4 ton SRW for a 1 ton DRW, if we added 300lbs?
Seriously, the best I recollect from physics class, you would have the fulcrum 60 inch in on a 200 inch run (the midpoint of the add'l weight). Hence 70% of the mechanical advantage is between the fulcrum and wheels, meaning the wheels are carrying 30% of the weight with 70% borne by the hitch.
Therefore 300lbs x (100%-70%) = 90 lbs to the wheels and 210lbs to the hitch. What the heck. Old Coots formula may be easier to understand.