If you are already 3.5" lower in the front, that's about as far as I'd lower it. I think its odd that they would say 1.5" above the ball prior to loading, as different vehicles have different spring packs and settle differently than others. Maybe they just recommend that as a starting point.
Just curious, what truck and what trailer? If you have big enough truck combined with small enough trailer you may not even "need" WDH. Assuming you have a front compartment, you might want to load up some heavier items there to increase tongue weight. I've got my tools, a hydraulic jack, chairs, beach umbrella, wood blocks for the stab jacks, small air compressor, etc up in the front compartment. That extra weight there helps with towing stability.