Adjusting the angle of the hitch head, will also change the angle of the spring bars when they are rest. To add more tension to the bars, you can use more or less links of chain at the hangers or adjust the head angle to do the same.
You want to allow as much chain as possible between the spring bars and tongue hangers to ease stress during turns, but you do not want the spring bars to be hanging so low that they drag on uneven ground. A general rule of thumb is to get the bars parallel with the tongue - This should give sufficient chain length and ground clearance.
My enclosed trailer is around 8000 lbs with 1000 lbs tongue weight, I use the maximum hitch angle in my application to redistribute that hitch weight between all the axles.